Derivative Financial Instruments Narrative (Details)	12 Months Ended
	Dec. 31, 2015 USD ($)	Sep. 30, 2018 USD ($) contracts	Apr. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Net unrealized loss on hedging instruments		$ 185,246,000		$ 170,478,000
Total debt		651,875,000
Debt Instrument, interest rate, stated percentage			1.38%
Secured Debt [Member]
Total debt		1,051,875,000		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		3
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		$ 2,100,000
Interest Rate Cap [Member]
Number of interest rate derivatives held		3
Derivative, notional amount		$ 100,000,000
Interest rate cash flow hedge gain (loss) to be reclassified during next 12 months, net		$ (100,000)
Derivative purchase price	$ 1,700,000
Derivative, cap interest rate		2.00%
Currency Swap [Member]
Derivative, notional amount			$ 56,000,000.0
Foreign currency cash flow hedge gain (loss) to be reclassified during next 12 months			$ 1,700,000
Forward Contracts [Member]
Number of foreign exchange contracts | contracts		52
Foreign Exchange Contract [Member] | Not Designated as Hedging Instrument [Member] | Minimum [Member]
Derivative, notional amount		$ 183,000
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		$ 652,000,000.0