Debt - Unsecured Senior Notes (Details) (USD $)	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Debt Instrument [Line Items]
Amortization of deferred financing costs	$ 2,400,000us-gaap_AmortizationOfFinancingCosts	$ 1,100,000us-gaap_AmortizationOfFinancingCosts
Convertible Debt [Member] \| Exchangeable Senior Notes due August 15, 2019 [Member]
Debt Instrument [Line Items]
Stated Rate	2.625%
Interest expense, debt	2,400,000
Contractual interest expense	1,600,000
Accretion of debt discount	700,000
Amortization of deferred financing costs	100,000
Senior Notes [Member] \| Series A, Series B, and Series C Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Debt instrument face amount	350,000,000
Senior Notes [Member] \| Series A Senior Notes due 2025 [Member]
Debt Instrument [Line Items]
Stated Rate	3.93%
Debt instrument face amount	100,000,000
Senior Notes [Member] \| Series B Senior Notes due 2027 [Member]
Debt Instrument [Line Items]
Stated Rate	4.09%
Debt instrument face amount	125,000,000
Senior Notes [Member] \| Series C Senior Notes due 2030 [Member]
Debt Instrument [Line Items]
Stated Rate	4.18%
Debt instrument face amount	$ 125,000,000

Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

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Amount of noncash expense included in interest expense to issue debt and obtain financing associated with the related debt instruments. Alternate captions include noncash interest expense.

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Face (par) amount of debt instrument at time of issuance.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Amount of the cost of borrowed funds accounted for as interest expense for debt.

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Represents the portion of interest incurred in the period on debt arrangements that was charged against earnings, excluding amortization of debt discount (premium) and financing costs.

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