Summary of Changes in Stockholders Equity (Parenthetical) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017
Equity [Abstract]
Cumulative effect of change in accounting principle, taxes					$ 13,079
Amount per Share of Common Stock	$ 0.32	$ 0.32	$ 0.29	$ 0.29	$ 0.64	$ 0.58
Record Date	Jun. 08, 2018	Mar. 08, 2018	Jun. 08, 2017	Mar. 08, 2017
Payable Date	Jun. 22, 2018	Mar. 22, 2018	Jun. 22, 2017	Mar. 20, 2017

Aggregate dividends declared during the period for each share of common stock outstanding.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
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Amount of the related income tax effect of a cumulative effect on retained earnings.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
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Date the declared dividend will be paid, in CCYY-MM-DD format.

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-Publisher FASB
-Name Accounting Standards Codification
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Date the holder must own the stock to be entitled to the dividend, in CCYY-MM-DD format.

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