The total amount of dividends declared in the period for each class of stock and the contractual amount of dividends (or interest on participating income bonds) that must be paid for the period (for example, unpaid cumulative dividends). Dividends declared in the current period do not include dividends declared in respect of prior-period unpaid cumulative dividends. Preferred dividends that are cumulative only if earned are deducted only to the extent that they are earned.

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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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- Definition

The earnings attributable to each security on the basis that all of the earnings for the period had been distributed (to the extent that each security may share in earnings). This is the earnings amount which reflects the participation rights in undistributed earnings of each security.

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- Definition

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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Earnings Per Share - Schedule of Basic Earnings Per Common 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, Basic, by Common Class, Including Two Class Method [Line Items]
Distributed earnings to common shareholders									$ 43,623us-gaap_DistributedEarnings	$ 39,685us-gaap_DistributedEarnings	$ 39,349us-gaap_DistributedEarnings
Undistributed earnings to common shareholders									60,175us-gaap_UndistributedEarnings	24,212us-gaap_UndistributedEarnings	35,603us-gaap_UndistributedEarnings
Total earnings to common shareholders									103,798	63,897	74,952
Distributed earnings per common share									$ 1.40	$ 1.37	$ 1.36
Undistributed earnings per common share									$ 1.92	$ 0.83	$ 1.23
Total earnings per common share	$ 1.08us-gaap_EarningsPerShareBasic	$ 0.93us-gaap_EarningsPerShareBasic	$ 0.53us-gaap_EarningsPerShareBasic	$ 0.75us-gaap_EarningsPerShareBasic	$ 0.86us-gaap_EarningsPerShareBasic	$ 0.78us-gaap_EarningsPerShareBasic	$ 0.53us-gaap_EarningsPerShareBasic	$ 0.02us-gaap_EarningsPerShareBasic	$ 3.32us-gaap_EarningsPerShareBasic	$ 2.20us-gaap_EarningsPerShareBasic	$ 2.59us-gaap_EarningsPerShareBasic
Unvested Restricted Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Distributed earnings to common shareholders									694	749	758
Undistributed earnings to common shareholders									958	457	685
Total earnings allocated to unvested restricted stock									$ 1,652	$ 1,206	$ 1,443
Distributed earnings per common share									$ 1.34	$ 1.35	$ 1.37
Undistributed earnings per common share									$ 1.85	$ 0.83	$ 1.24
Total earnings per common share									$ 3.19	$ 2.18	$ 2.61