Debt - Unsecured Senior Notes (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Aug. 31, 2014
Debt Instrument [Line Items]
Amortization of deferred financing costs	$ 1,500,000	$ 1,400,000	$ 3,900,000	$ 2,500,000
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%
Interest expense, debt	$ 2,500,000		$ 4,900,000
Contractual interest expense	1,700,000		3,300,000
Accretion of debt discount	600,000		1,300,000
Amortization of deferred financing costs	200,000		300,000
Senior Notes | Series A, Series B, and Series C Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Debt instrument face amount	350,000,000		350,000,000
Senior Notes | Series A Senior Notes due 2025
Debt Instrument [Line Items]
Debt instrument face amount	$ 100,000,000		$ 100,000,000
Stated Rate	3.93%		3.93%
Senior Notes | Series B Senior Notes due 2027
Debt Instrument [Line Items]
Debt instrument face amount	$ 125,000,000		$ 125,000,000
Stated Rate	4.09%		4.09%
Senior Notes | Series C Senior Notes due 2030
Debt Instrument [Line Items]
Debt instrument face amount	$ 125,000,000		$ 125,000,000
Stated Rate	4.18%		4.18%