Annual and transition report of foreign private issuers pursuant to Section 13 or 15(d)

Debt (Junior subordinated notes) (Details)

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Debt (Junior subordinated notes) (Details) - USD ($)
1 Months Ended 12 Months Ended
Jun. 30, 2015
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]      
Debt   $ 28,420,739,000 $ 27,716,999,000
Revolving Credit Facility Before Issuance Of Junior Subondinated Notes [Member]      
Debt Instrument [Line Items]      
Credit facility maximum borrowing capacity $ 1,000,000,000.0    
Revolving Credit Facility After Issuance Of Junior Subondinated Notes [Member]      
Debt Instrument [Line Items]      
Credit facility maximum borrowing capacity $ 500,000,000.0    
Deferral of interest payments, maximum term 5 years    
Subordinated Debt [Member] | Junior Subordinated Notes [Member]      
Debt Instrument [Line Items]      
Debt $ 500,000,000    
Debt, fixed interest rate 6.50%    
Spread over reference rate 4.30%    
Subordinated Debt [Member] | Junior Subordinated Notes [Member] | Debt Instrument, Redemption, Option (i) [Member]      
Debt Instrument [Line Items]      
Redemption price of debt instrument, percentage   100.00%  
Subordinated Debt [Member] | Junior Subordinated Notes [Member] | Debt Instrument, Redemption, Option (iii) [Member]      
Debt Instrument [Line Items]      
Redemption price of debt instrument, percentage   100.00%  
Subordinated Debt [Member] | Junior Subordinated Notes [Member] | Debt Instrument, Redemption, Option (iv) [Member]      
Debt Instrument [Line Items]      
Redemption price of debt instrument, percentage   101.00%  
Days after occurence   60 days  
Annual interest rate increase   5.00%