Derivative Financial Instruments Narrative (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) contracts	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Total debt	$ 745,777,000
Net unrealized loss on hedging instruments	152,595,000		$ 170,478,000
Secured Debt [Member]
Total debt	1,145,777,000		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	3		3
Interest Rate Swap [Member]
Number of Interest Rate Derivatives Held | contracts	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	$ 1,300,000
Interest Rate Cap [Member]
Number of Interest Rate Derivatives Held | contracts	3
Derivative, Notional Amount	$ 100,000,000
Derivative, Cap Interest Rate	2.00%
Derivative Purchase Price		$ 1,700,000
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	$ (300,000)
Forward Contracts [Member]
Description of Derivative Activity Volume	57
Foreign Exchange Contract [Member] | Not Designated as Hedging Instrument [Member] | Minimum [Member]
Derivative, Notional Amount	$ 93,000
Foreign Exchange Contract [Member] | Not Designated as Hedging Instrument [Member] | Maximum [Member]
Derivative, Notional Amount	47,500,000
London Interbank Offered Rate (LIBOR) [Member] | Secured Debt [Member]
Total debt	$ 690,000,000