Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Net income	$ 175,722	$ 163,356	$ 151,742
Other comprehensive income before income taxes
Net change in unrealized (loss) gain	(26,311)	(22,772)	8,140
Reclassification adjustment for net losses realized and included in earnings	390	8,527	6,418
Accretion (amortization) of unrealized net gain (loss) on securities transferred to held to maturity	3,297	(6,371)	(8,752)
Other comprehensive (loss) income before income taxes	(22,624)	(20,616)	5,806
Income tax (benefit) expense	(7,929)	(8,162)	2,221
Other comprehensive (loss) income net of income taxes	(14,695)	(12,454)	3,585
Comprehensive income	$ 161,027	$ 150,902	$ 155,327