Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 242,135	$ 240,716	$ 231,139	$ 224,114	$ 224,703	$ 226,419	$ 225,453	$ 234,064	$ 938,104	$ 910,639	$ 929,866
Interest expense	21,514	20,252	19,061	17,389	16,907	16,527	16,255	16,362	78,216	66,051	58,232
Net interest income	220,621	220,464	212,078	206,725	207,796	209,892	209,198	217,702	859,888	844,588	871,634
Provision for loan and lease losses	12,928	11,997	10,657	11,672	13,171	13,091	10,589	4,823	47,254	41,674	36,589
Non-interest income	70,450	75,402	71,119	60,225	98,620	80,710	74,659	45,951	277,196	299,940	275,724
Non-interest expense	192,786	188,354	184,021	182,714	183,468	181,187	188,511	183,989	747,875	737,155	763,642
Income before income taxes	85,357	95,515	88,519	72,564	109,777	96,324	84,757	74,841	341,955	365,699	347,127
Provision for income taxes	3,486	34,182	31,707	26,561	40,502	34,515	30,470	27,272	95,936	132,759	124,588
Net income	81,871	61,333	56,812	46,003	69,275	61,809	54,287	47,569	246,019	232,940	222,539
Dividends and undistributed earnings allocated to participating securities	16	14	14	12	33	31	32	29	56	125	357
Net earnings available to common shareholders	$ 81,855	$ 61,319	$ 56,798	$ 45,991	$ 69,242	$ 61,778	$ 54,255	$ 47,540	$ 245,963	$ 232,815	$ 222,182
Basic earnings per common share (usd per share)	$ 0.37	$ 0.28	$ 0.26	$ 0.21	$ 0.31	$ 0.28	$ 0.25	$ 0.22	$ 1.12	$ 1.06	$ 1.01
Diluted earnings per common share (usd per share)	0.37	0.28	0.26	0.21	0.31	0.28	0.25	0.22	$ 1.11	$ 1.05	$ 1.01
Cash dividends declared per common share (usd per share)	$ 0.18	$ 0.18	$ 0.16	$ 0.16	$ 0.16	$ 0.16	$ 0.16	$ 0.16