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

Derivative assets (Income (loss) recorded in comprehensive income) (Details)

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Derivative assets (Income (loss) recorded in comprehensive income) (Details) - Interest Rate Swap [Member] - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]      
Effective portion of change in fair market value of derivatives $ 17,049 $ 6,846 $ 385
Net changes in cash flow hedges, Income tax effect (2,131) (856) (47)
Net change in fair value of derivatives, net of tax $ 14,918 $ 5,990 $ 338