Earnings per Share - Schedule of Earnings per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Basic earnings per share, Net Income									$ 49,919	$ 51,920	$ 74,221
Diluted earnings per share, Net Income									$ 49,919	$ 51,920	$ 74,221
Basic earnings per share, Shares									87,183	87,537	82,480
Effect of dilutive incentive-based awards, Shares									297	615	1,072
Diluted earnings per share, Shares									87,480	88,152	83,552
Basic earnings per share, Per Share Amount	$ (0.29)us-gaap_EarningsPerShareBasic	$ 1.01us-gaap_EarningsPerShareBasic	$ 0.43us-gaap_EarningsPerShareBasic	$ (0.56)us-gaap_EarningsPerShareBasic	$ (0.14)us-gaap_EarningsPerShareBasic	$ 1.35us-gaap_EarningsPerShareBasic	$ (0.18)us-gaap_EarningsPerShareBasic	$ (0.48)us-gaap_EarningsPerShareBasic	$ 0.57us-gaap_EarningsPerShareBasic	$ 0.59us-gaap_EarningsPerShareBasic	$ 0.90us-gaap_EarningsPerShareBasic
Diluted earnings per share, Per Share Amount	$ (0.29)us-gaap_EarningsPerShareDiluted	$ 1.00us-gaap_EarningsPerShareDiluted	$ 0.43us-gaap_EarningsPerShareDiluted	$ (0.56)us-gaap_EarningsPerShareDiluted	$ (0.14)us-gaap_EarningsPerShareDiluted	$ 1.34us-gaap_EarningsPerShareDiluted	$ (0.18)us-gaap_EarningsPerShareDiluted	$ (0.48)us-gaap_EarningsPerShareDiluted	$ 0.57us-gaap_EarningsPerShareDiluted	$ 0.59us-gaap_EarningsPerShareDiluted	$ 0.89us-gaap_EarningsPerShareDiluted

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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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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 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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