Business Segments (Operating Results and Other Key Financial Measures) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment Reporting Information
Interest income	$ 371,873	$ 302,669
Charge for funds used	(178,966)	(120,414)
Interest spread on funds used	192,907	182,255
Interest expense	(45,180)	(30,547)
Credit on funds provided	178,966	120,414
Interest spread on funds provided (used)	133,786	89,867
Net interest income before provision for credit losses	326,693	272,122
Provision for credit losses	20,218	7,068
Noninterest income	74,444	115,828
Noninterest expense	169,135	152,878
Segment income (loss) before income taxes	211,784	228,004
Segment income after income taxes	187,032	169,736
Segment assets	37,693,158	35,342,126	$ 37,150,249
Retail Banking
Segment Reporting Information
Interest income	104,710	81,025
Charge for funds used	(49,273)	(27,738)
Interest spread on funds used	55,437	53,287
Interest expense	(24,940)	(16,183)
Credit on funds provided	145,451	102,546
Interest spread on funds provided (used)	120,511	86,363
Net interest income before provision for credit losses	175,948	139,650
Provision for credit losses	3,093	378
Noninterest income	44,448	13,564
Noninterest expense	81,968	72,844
Segment income (loss) before income taxes	135,335	79,992
Segment income after income taxes	96,968	47,035
Segment assets	9,345,892	8,213,268
Commercial Banking
Segment Reporting Information
Interest income	239,577	192,419
Charge for funds used	(111,366)	(64,509)
Interest spread on funds used	128,211	127,910
Interest expense	(9,179)	(5,098)
Credit on funds provided	25,448	12,043
Interest spread on funds provided (used)	16,269	6,945
Net interest income before provision for credit losses	144,480	134,855
Provision for credit losses	17,125	6,690
Noninterest income	27,438	25,734
Noninterest expense	65,020	54,373
Segment income (loss) before income taxes	89,773	99,526
Segment income after income taxes	64,362	58,796
Segment assets	21,992,393	19,624,237
Other
Segment Reporting Information
Interest income	27,586	29,225
Charge for funds used	(18,327)	(28,167)
Interest spread on funds used	9,259	1,058
Interest expense	(11,061)	(9,266)
Credit on funds provided	8,067	5,825
Interest spread on funds provided (used)	(2,994)	(3,441)
Net interest income before provision for credit losses	6,265	(2,383)
Provision for credit losses	0	0
Noninterest income	2,558	76,530
Noninterest expense	22,147	25,661
Segment income (loss) before income taxes	(13,324)	48,486
Segment income after income taxes	25,702	63,905
Segment assets	$ 6,354,873	$ 7,504,621