Summary of Quarterly Financial Data (Unaudited) (Schedule) (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 Data [Abstract]
Interest income	$ 306,934	$ 297,652	$ 285,510	$ 272,401	$ 264,534	$ 256,554	$ 252,393	$ 249,323	$ 1,162,497	$ 1,022,803	$ 945,962
Net interest income	269,713	262,572	251,097	239,927	233,530	226,007	221,449	218,193	1,023,309	899,180	827,318
Provision for loan losses	8,565	39,686	10,260	8,674	6,259	5,671	6,693	9,377	67,185	28,000	19,010
Income before income taxes	103,966	152,675	117,791	105,704	108,068	102,620	94,031	83,730	480,138	388,451	358,573
Income tax expense	74,361	54,668	41,788	33,847	39,519	37,375	33,574	31,199	204,664	141,667	132,491
Net income	29,605	98,007	76,003	71,857	68,549	65,245	60,457	52,531	275,474	246,784	226,082
Net income available common shareholders	$ 27,046	$ 95,448	$ 73,444	$ 69,298	$ 65,990	$ 62,686	$ 57,898	$ 49,972	$ 265,236	$ 236,546	$ 215,844
Net income per common share, basic (in dollars per share)	$ 0.23	$ 0.79	$ 0.60	$ 0.57	$ 0.54	$ 0.51	$ 0.46	$ 0.39	$ 2.19	$ 1.90	$ 1.63
Net income per common share, diluted (in dollars per share)	$ 0.23	$ 0.78	$ 0.60	$ 0.56	$ 0.54	$ 0.51	$ 0.46	$ 0.39	$ 2.17	$ 1.89	$ 1.62