Derivative Instruments and Hedging Activities (Details)	1 Months Ended	12 Months Ended
	Nov. 30, 2016 USD ($) instrument	Dec. 31, 2017 USD ($) instrument	Dec. 31, 2016 USD ($) instrument	Dec. 31, 2015 USD ($)	Oct. 31, 2016 USD ($)
Derivative [Line Items]
Unsecured debt, net		$ 3,501,709,000	$ 3,246,779,000
Total return swap income		10,098,000	11,716,000	$ 5,655,000
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Hedge ineffectiveness related to cash flow hedges		$ 100,000	$ 300,000	0
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Number of derivative instruments held | instrument	4	5	1
Average fixed interest rate		2.30%
Derivative, fair value		$ 5,400,000	$ 4,400,000
Aggregate carrying value of the interest rate swap contracts		0	30,000
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Total return swap income		10,100,000	11,700,000	$ 5,700,000
Not Designated as Hedging Instrument [Member] | Interest Rate Cap [Member]
Derivative [Line Items]
Derivative, notional amount		20,700,000
Derivative, fair value		0	0
Not Designated as Hedging Instrument [Member] | Total Return Swap [Member]
Derivative [Line Items]
Derivative, fair value		0
Not Designated as Hedging Instrument [Member] | Total Return Swap [Callable]
Derivative [Line Items]
Derivative, notional amount		$ 256,624,000
Variable rate term loan [Member]
Derivative [Line Items]
Unsecured debt, net	$ 350,000,000		$ 100,000,000		$ 225,000,000
Debt instrument, term	5 years
Basis spread on rate		0.95%
Derivative, notional amount		$ 175,000,000
Variable rate term loan [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Unsecured debt, net		$ 350,000,000
Number of derivative instruments held | instrument		5