Parent Company Information - Schedule of Condensed Statements of Income (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
Gains on securities transactions, net	$ 643	$ 103	$ 7	$ (8)	$ 10,670	$ 9	$ 41	$ 3,958	$ 745	$ 14,678	$ 2,587
Trading (losses) gains, net	47	(35)	(34)	(9)	1,540	2,729	14,366	15,060	(31)	909	2,793
Income tax benefit	7,827	10,654	11,751	830	16,061	7,143	10,961	12,814	31,062	46,979	66,748
Net Income	25,135	27,682	29,520	33,835	39,608	27,121	33,922	31,310	116,172	131,961	143,627
Parent Company [Member]
Dividends from subsidiary									110,000	120,000	145,000
Income from subsidiary									0	6	53
Gains on securities transactions, net									0	14	0
Trading (losses) gains, net									0	881	2,574
Other interest and dividends									436	576	922
Total Income									110,436	121,477	148,549
Total Expenses									11,172	16,140	17,040
Income before income tax benefit and equity in undistributed earnings of subsidiary									99,264	105,337	131,509
Income tax benefit									(3,245)	(5,130)	(4,737)
Income before equity in undistributed earnings of subsidiary									102,509	110,467	136,246
Equity in undistributed earnings of subsidiary									13,663	21,494	7,381
Net Income									$ 116,172	$ 131,961	$ 143,627