Derivative Financial Instruments Narrative (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($)	Dec. 31, 2015 USD ($)	Apr. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Net unrealized loss on hedging instruments	$ 185,868,000			$ 170,478,000
Total debt	741,938,000
Debt Instrument, Interest Rate, Stated Percentage			1.38%
Secured Debt [Member]
Total debt	1,141,938,000			619,477,000
Accumulated Other Comprehensive Income, Net Unrealized Gain (Loss) on Nonderivative Instruments [Member]
Net unrealized loss on hedging instruments	$ 14,380,000			$ 14,380,000	$ 14,380,000	$ 14,380,000
Number of Counterparties [Member]
Derivative Counterparties	5			5
Interest Rate Swap [Member]
Number of Interest Rate Derivatives Held	1
Derivative, Notional Amount	$ 214,000,000
Pay fixed interest rate in interest rate swap	1.42%
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	$ 1,700,000
Interest Rate Cap [Member]
Number of Interest Rate Derivatives Held	3
Derivative, Notional Amount	$ 100,000,000
Derivative, Cap Interest Rate	2.00%
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	$ (100,000)
Derivative Purchase Price		$ 1,700,000
Currency Swap [Member]
Derivative, Notional Amount			$ 56,000,000
Foreign Currency Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months			$ 1,500,000
Forward Contracts [Member]
Description of Derivative Activity Volume	57
Foreign Exchange Contract [Member] | Not Designated as Hedging Instrument [Member] | Minimum [Member]
Derivative, Notional Amount	$ 133,000
Foreign Exchange Contract [Member] | Not Designated as Hedging Instrument [Member] | Maximum [Member]
Derivative, Notional Amount	55,500,000
London Interbank Offered Rate (LIBOR) [Member] | Secured Debt [Member]
Total debt	$ 742,000,000