Net Income Per Share - Calculation of Net Income Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Earnings Per Share [Abstract]
Net income	$ 85,184	$ 66,358	$ 221,875	$ 185,310
Income allocated to participating securities-basic	0	0	0	3
Net income available to common stockholders-basic	85,184	66,358	221,875	185,307
Basic weighted average shares	134,054	135,727	134,690	136,469
Basic net income per share	$ 0.64	$ 0.49	$ 1.65	$ 1.36
Net income	85,184	66,358	221,875	185,310
Income allocated to participating securities-diluted	0	0	0	3
Net income available to common stockholders—diluted	$ 85,184	$ 66,358	$ 221,875	$ 185,307
Basic weighted average shares	134,054	135,727	134,690	136,469
Dilutive effect of potential common shares	1,312	1,514	1,050	1,290
Diluted weighted average shares	135,366	137,241	135,740	137,759
Diluted net income per share	$ 0.63	$ 0.48	$ 1.63	$ 1.35

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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Net Income or Loss Available to Common Stockholders plus adjustments resulting from the assumption that dilutive convertible securities were converted, options or warrants were exercised, or that other shares were issued upon the satisfaction of certain conditions.

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The earnings that is allocated to common stock and participating securities to the extent that each security may share in earnings as if all of the earnings for the period had been distributed.

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The sum of dilutive potential common shares or units used in the calculation of the diluted per-share or per-unit computation.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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