Shareholders' Equity - Schedule of Cash Dividends Declared Per Common Share and Total Amounts Declared, Distributed and Deferred (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement Of Stockholders Equity [Abstract]
Cash dividends declared, Per share	$ 0.28	$ 0.22	$ 0.22	$ 0.22	$ 0.18	$ 0.18	$ 0.72	$ 0.58
Cash dividends declared, Amount	$ 26,936	$ 21,588	$ 22,046	$ 24,210	$ 20,443	$ 20,424	$ 70,570	$ 65,077
Cash dividends distributed, Amount	26,680	21,391	21,889	24,152	20,441	20,411	69,960	65,004
Cash Dividends Deferred, Amount	$ 256	$ 197	$ 157	$ 58	$ 2	$ 13	$ 610	$ 73