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

Debt (Subordinated debt in joint venture partners) (Details)

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Debt (Subordinated debt in joint venture partners) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Jan. 01, 2013
Dec. 31, 2010
Debt Instrument [Line Items]        
Debt $ 28,420,739 $ 27,716,999    
Subordinated Debt Joint Ventures Partners [Member]        
Debt Instrument [Line Items]        
Stated interest rate     0.00%  
Interest rate paid on unpaid interest 19.50%      
Subordinated Debt Joint Ventures Partners Before Amendment [Member]        
Debt Instrument [Line Items]        
Stated interest rate 20.00%      
Minimum [Member] | Subordinated Debt Joint Ventures Partners [Member]        
Debt Instrument [Line Items]        
Stated interest rate 15.00%      
Subordinated Debt [Member] | Subordinated Debt Joint Ventures Partners [Member]        
Debt Instrument [Line Items]        
Debt       $ 64,300