Debt - Senior Unsecured Notes (Details)	1 Months Ended	12 Months Ended
	Aug. 31, 2014 USD ($) d $ / shares	Dec. 31, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Mar. 31, 2015 USD ($)
Debt Instrument [Line Items]
Proceeds from mortgage notes payable		$ 50,000,000	$ 0	$ 191,000,000
Senior unsecured notes		590,388,000	587,018,000
Interest expense		71,147,000	67,492,000	66,456,000
Amortization expense of deferred financing costs		$ 5,000,000	6,100,000	7,600,000
Convertible debt [Member] | Exchangeable Senior Notes due August 15, 2019 [Member]
Debt Instrument [Line Items]
Face amount	$ 250,000,000
Stated interest rate	2.625%	2.625%
Proceeds from mortgage notes payable	$ 246,900,000
Conversion price (USD per share) | $ / shares	$ 19.45	$ 19.39
Percentage of principal redeemed	100.00%
Effective Rate	3.80%	3.93%
Senior unsecured notes	$ 236,600,000
Embedded conversion option		$ 13,400,000	13,400,000
Remaining discount amortization period	5 years
Unamortized discount		7,000,000	9,700,000
Underwriting expense	$ 3,100,000
Deferred finance costs	$ 2,900,000
Embedded conversion feature, reduction		$ 200,000	200,000
Debt, conversion ratio	0.0514059	0.0515600
Interest on note		$ 9,900,000	9,900,000	3,800,000
Interest expense		6,600,000	6,600,000	2,500,000
Accretion of debt discount		2,700,000	2,700,000	1,100,000
Amortization expense of deferred financing costs		$ 600,000	$ 600,000	$ 200,000
Senior Notes [Member] | Series A, Series B, and Series C Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Face amount					$ 350,000,000
Senior Notes [Member] | Series A Senior Notes due 2025 [Member]
Debt Instrument [Line Items]
Face amount					$ 100,000,000
Stated interest rate		3.93%			3.93%
Effective Rate		4.03%
Senior Notes [Member] | Series B Senior Notes due 2027 [Member]
Debt Instrument [Line Items]
Face amount					$ 125,000,000
Stated interest rate		4.09%			4.09%
Effective Rate		4.19%
Senior Notes [Member] | Series C Senior Notes due 2030 [Member]
Debt Instrument [Line Items]
Face amount					$ 125,000,000
Stated interest rate		4.18%			4.18%
Effective Rate		4.28%
Covenant scenario one [Member] | Convertible debt [Member] | Exchangeable Senior Notes due August 15, 2019 [Member]
Debt Instrument [Line Items]
Threshold percentage of stock price trigger	130.00%
Threshold trading days | d	20
Threshold consecutive trading days	30 days
Covenant scenario two [Member] | Convertible debt [Member] | Exchangeable Senior Notes due August 15, 2019 [Member]
Debt Instrument [Line Items]
Threshold percentage of stock price trigger	98.00%
Threshold consecutive trading days	5 days
Threshold consecutive business days	5 days