Parent Company Information - Schedule of Condensed Statements of Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Gains on securities transactions, net	$ 10,670	$ 9	$ 41	$ 3,958	$ 44	$ 1,496	$ 1,204	$ (157)	$ 14,678	$ 2,587	$ 32,068
Trading gains, net	1,150	2,231	(270)	(2,202)	2,166	6	1,609	(988)	909	2,793	2,271
Income tax benefit	16,061	7,143	10,961	12,814	14,702	22,402	14,366	15,278	46,979	66,748	62,706
Net income	39,608	27,121	33,922	31,310	36,829	39,447	32,820	34,531	131,961	143,627	132,511
Parent Company [Member]
Dividends from subsidiary									120,000	145,000	115,000
Income from subsidiary									6	53	180
Gains on securities transactions, net									14
Trading gains, net									881	2,574	1,256
Other interest and dividends									576	922	223
Total Income									121,477	148,549	116,659
Total Expenses									16,140	17,040	16,481
Income before income tax benefit and equity in undistributed earnings of subsidiary									105,337	131,509	100,178
Income tax benefit									(5,130)	(4,737)	(5,202)
Income before equity in undistributed earnings of subsidiary									110,467	136,246	105,380
Equity in undistributed earnings of subsidiary									21,494	7,381	27,131
Net income									$ 131,961	$ 143,627	$ 132,511