Parent Company Financial Information - Condensed Income Statements (Detail) (USD $) In Thousands, 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
Condensed Financial Statements, Captions [Line Items]
Non-interest income	$ 8,607	$ 6,226	$ 5,773	$ 4,835	$ 1,557	$ 4,129	$ 11,762	$ 4,799	$ 25,441	$ 22,247	$ 46,505
Expenses:
Interest expense	8,006	7,481	8,075	7,924	7,601	8,121	7,133	6,905	31,486	29,760	28,032
Non-interest expense	55,932	50,012	52,416	49,749	51,131	49,675	48,531	46,929	208,109	196,266	188,860
Income before income taxes and equity in undistributed earnings of subsidiaries	54,520	53,868	46,748	42,363	36,107	39,013	41,902	28,634	197,499	145,656	101,064
Income tax benefit (expense)	(14,111)	(12,949)	(10,706)	(10,624)	(3,992)	(10,390)	(7,661)	(7,787)	(48,390)	(29,830)	(25,935)
Net income	40,409	40,919	35,538	31,085	31,414	28,594	34,072	20,885	147,951	114,965	72,639
Dividends on preferred stock	329	353	352	353	352	352	353	353	1,387	1,410	3,793
Net income available to common shareholders	40,080	40,566	35,186	30,732	31,062	28,242	33,719	20,532	146,564	113,555	68,846
Consolidated Company [Member]
Condensed Financial Statements, Captions [Line Items]
Dividends from subsidiaries									67,515	71,529	18,499
Interest income									4,381	2,847	2,105
Non-interest income									1,737	3,995	27,563
Total income									73,633	78,371	48,167
Expenses:
Interest expense									8,776	10,833	10,522
Non-interest expense									10,850	32,001	30,064
Total expense									19,626	42,834	40,586
Income before income taxes and equity in undistributed earnings of subsidiaries									54,007	35,537	7,581
Income tax benefit (expense)									5,388	21,426	11,353
Income before equity in undistributed earnings of subsidiaries									59,395	56,963	18,934
Equity in undistributed earnings of subsidiaries									88,556	58,002	53,705
Net income									147,951	114,965	72,639
Dividends on preferred stock									1,387	1,410	3,793
Net income available to common shareholders									$ 146,564	$ 113,555	$ 68,846