Debt - Secured Unsecured Notes (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Mar. 31, 2015	Aug. 31, 2014
Debt Instrument [Line Items]
Amortization of deferred financing costs	$ 1,300,000	$ 1,100,000	$ 3,700,000	$ 5,000,000
Convertible Debt | Exchangeable Senior Notes due August 15, 2019
Debt Instrument [Line Items]
Debt instrument face amount						$ 250,000,000.0
Stated rate	2.625%		2.625%			2.625%
Shares converted (in shares)			51.5095
Conversion price (in USD per share)						$ 19.41
Interest expense, debt	$ 2,400,000	2,400,000	$ 7,300,000	7,300,000
Contractual interest expense	1,600,000	1,600,000	4,900,000	4,900,000
Accretion of debt discount	700,000	700,000	2,000,000	2,000,000
Amortization of deferred financing costs	$ 100,000	$ 100,000	$ 400,000	$ 400,000
Senior Notes | Series A, Series B, and Series C Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Debt instrument face amount					$ 350,000,000
Senior Notes | Series A Senior Notes due 2025
Debt Instrument [Line Items]
Debt instrument face amount					$ 100,000,000
Stated rate	3.93%		3.93%		3.93%
Senior Notes | Series B Senior Notes due 2027
Debt Instrument [Line Items]
Debt instrument face amount					$ 125,000,000
Stated rate	4.09%		4.09%		4.09%
Senior Notes | Series C Senior Notes due 2030
Debt Instrument [Line Items]
Debt instrument face amount					$ 125,000,000
Stated rate	4.18%		4.18%		4.18%