Summary of Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Data [Abstract]
Interest income	$ 242,814	$ 238,093	$ 233,654	$ 231,401	$ 234,703	$ 233,394	$ 232,213	$ 228,382	$ 945,962	$ 928,692	$ 929,014
Net interest income	212,620	207,790	203,644	203,263	207,456	206,263	205,051	200,514	827,318	819,284	810,192
Provision for loan losses	5,021	2,956	6,636	4,397	8,193	3,843	12,284	9,511	19,010	33,831	69,598
Income before income taxes	90,741	93,986	88,034	85,812	78,929	72,656	73,950	77,024	358,573	302,559	252,628
Income tax expense	32,342	36,058	32,242	31,849	25,756	25,868	27,078	28,608	132,491	107,310	93,245
Net income	58,398	57,928	55,792	53,963	53,173	46,788	46,872	48,416	226,082	195,249	159,383
Net income available common shareholders	$ 55,839	$ 55,369	$ 53,233	$ 51,404	$ 50,612	$ 44,229	$ 44,313	$ 45,857	$ 215,844	$ 185,011	$ 118,553
Net income per common share, basic (per share)	$ 0.43	$ 0.42	$ 0.40	$ 0.38	$ 0.37	$ 0.32	$ 0.32	$ 0.33	$ 1.63	$ 1.34	$ 0.93
Net income per common share, diluted (per share)	$ 0.43	$ 0.42	$ 0.40	$ 0.38	$ 0.37	$ 0.32	$ 0.32	$ 0.33	$ 1.62	$ 1.33	$ 0.88
Allowance for loan losses and provision for loan losses	$ 12,900								$ 13,300