Business Segments (Operating Results and Other Key Financial Measures) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Segment Reporting Information
Interest income	$ 400,311	$ 322,775	$ 772,184	$ 625,444
Charge for funds used	(200,371)	(125,406)	(379,337)	(245,820)
Interest spread on funds used	199,940	197,369	392,847	379,624
Interest expense	(58,632)	(32,684)	(103,812)	(63,231)
Credit on funds provided	200,371	125,406	379,337	245,820
Interest spread on funds provided (used)	141,739	92,722	275,525	182,589
Net interest income before provision for credit losses	341,679	290,091	668,372	562,213
(Reversal of) provision for credit losses	15,536	10,685	35,754	17,753
Noninterest income	48,268	47,244	122,712	163,072
Noninterest expense	177,419	168,965	346,554	321,843
Segment income (loss) before income taxes	196,992	157,685	408,776	385,689
Segment income after income taxes	172,349	118,330	359,381	288,066
Segment assets	38,072,954	35,917,617	38,072,954	35,917,617	$ 37,150,249
Retail Banking
Segment Reporting Information
Interest income	110,808	88,752	215,518	169,777
Charge for funds used	(56,915)	(33,139)	(106,188)	(60,877)
Interest spread on funds used	53,893	55,613	109,330	108,900
Interest expense	(31,991)	(18,377)	(56,931)	(34,560)
Credit on funds provided	158,446	106,094	303,897	208,640
Interest spread on funds provided (used)	126,455	87,717	246,966	174,080
Net interest income before provision for credit losses	180,348	143,330	356,296	282,980
(Reversal of) provision for credit losses	3,414	(664)	6,507	(286)
Noninterest income	14,585	13,641	59,033	27,205
Noninterest expense	86,144	79,950	168,112	152,794
Segment income (loss) before income taxes	105,375	77,685	240,710	157,677
Segment income after income taxes	75,496	45,679	172,464	92,714
Segment assets	9,816,103	8,438,706	9,816,103	8,438,706
Commercial Banking
Segment Reporting Information
Interest income	261,567	205,873	501,144	398,292
Charge for funds used	(126,782)	(77,750)	(238,148)	(142,259)
Interest spread on funds used	134,785	128,123	262,996	256,033
Interest expense	(11,635)	(5,184)	(20,814)	(10,282)
Credit on funds provided	30,728	12,623	56,176	24,666
Interest spread on funds provided (used)	19,093	7,439	35,362	14,384
Net interest income before provision for credit losses	153,878	135,562	298,358	270,417
(Reversal of) provision for credit losses	12,122	11,349	29,247	18,039
Noninterest income	30,744	26,584	58,182	52,318
Noninterest expense	56,706	44,551	121,726	98,924
Segment income (loss) before income taxes	115,794	106,246	205,567	205,772
Segment income after income taxes	83,210	62,766	147,572	121,562
Segment assets	22,199,992	20,456,579	22,199,992	20,456,579
Other
Segment Reporting Information
Interest income	27,936	28,150	55,522	57,375
Charge for funds used	(16,674)	(14,517)	(35,001)	(42,684)
Interest spread on funds used	11,262	13,633	20,521	14,691
Interest expense	(15,006)	(9,123)	(26,067)	(18,389)
Credit on funds provided	11,197	6,689	19,264	12,514
Interest spread on funds provided (used)	(3,809)	(2,434)	(6,803)	(5,875)
Net interest income before provision for credit losses	7,453	11,199	13,718	8,816
(Reversal of) provision for credit losses	0	0	0	0
Noninterest income	2,939	7,019	5,497	83,549
Noninterest expense	34,569	44,464	56,716	70,125
Segment income (loss) before income taxes	(24,177)	(26,246)	(37,501)	22,240
Segment income after income taxes	13,643	9,885	39,345	73,790
Segment assets	$ 6,056,859	$ 7,022,332	$ 6,056,859	$ 7,022,332