Financial Instruments and Fair Values - Effect of Derivative Financial Instruments Designated as Cash Flow Hedges (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Interest expense	$ (20,525)	$ (17,477)	$ (38,116)	$ (35,219)
Designated as Hedging Instrument \| Cash Flow Hedging \| Interest Rate Swap
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in other comprehensive income (loss)	1,763	(14,410)	5,943	(13,016)
Amount of gain (loss) reclassified from accumulated other comprehensive income (loss) into interest expense	(536)	(43)	(1,135)	(43)
Reclassification out of Accumulated Other Comprehensive Income \| Accumulated net gain (loss) from cash flow hedges \| Interest Rate Swap
Derivative Instruments, Gain (Loss) [Line Items]
Interest expense	$ (536)	$ (43)	$ (1,135)	$ (43)

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

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Amount of the cost of borrowed funds accounted for as interest expense.

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Amount before tax of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

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