Business Segments (Operating Results and Other Key Financial Measures) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information
Interest income	$ 457,334	$ 422,185	$ 400,311	$ 371,873	$ 359,765	$ 339,910	$ 322,775	$ 302,669	$ 1,651,703	$ 1,325,119	$ 1,137,481
Charge for funds used									(835,106)	(533,777)	(360,465)
Interest spread on funds used									816,597	791,342	777,016
Interest expense	(87,918)	(73,465)	(58,632)	(45,180)	(40,064)	(36,755)	(32,684)	(30,547)	(265,195)	(140,050)	(104,843)
Credit on funds provided									835,106	533,777	360,465
Interest spread on funds provided									569,911	393,727	255,622
Net interest income before provision for credit losses	369,416	348,720	341,679	326,693	319,701	303,155	290,091	272,122	1,386,508	1,185,069	1,032,638
(Reversal of) provision for credit losses	17,959	10,542	15,536	20,218	15,517	12,996	10,685	7,068	64,255	46,266	27,479
Noninterest income	41,695	46,502	48,268	74,444	45,206	49,470	47,244	115,828	210,909	257,748	182,278
Noninterest expense	188,097	179,815	177,419	169,135	175,263	164,345	168,965	152,878	714,466	661,451	615,249
Segment income (loss) before income taxes	205,055	204,865	196,992	211,784	174,127	175,284	157,685	228,004	818,696	735,100	572,188
Segment net income	173,018	$ 171,302	$ 172,349	$ 187,032	84,898	$ 132,660	$ 118,330	$ 169,736	703,701	505,624	431,677
Segment assets	41,042,356				37,121,563				41,042,356	37,121,563	34,788,840
Consumer and Business Banking
Segment Reporting Information
Interest income									466,504	364,906	315,146
Charge for funds used									(245,487)	(142,619)	(95,970)
Interest spread on funds used									221,017	222,287	219,176
Interest expense									(149,032)	(76,770)	(60,180)
Credit on funds provided									655,230	445,304	300,446
Interest spread on funds provided									506,198	368,534	240,266
Net interest income before provision for credit losses									727,215	590,821	459,442
(Reversal of) provision for credit losses									9,364	1,812	(4,356)
Noninterest income									85,607	54,451	51,251
Noninterest expense									336,412	319,645	306,386
Segment income (loss) before income taxes									467,046	323,815	208,663
Segment net income									334,255	190,404	122,256
Segment assets	10,587,621				9,316,587				10,587,621	9,316,587	7,821,610
Commercial Banking
Segment Reporting Information
Interest income									1,063,658	844,303	726,013
Charge for funds used									(535,445)	(326,902)	(216,849)
Interest spread on funds used									528,213	517,401	509,164
Interest expense									(52,613)	(24,603)	(16,892)
Credit on funds provided									130,050	61,019	38,636
Interest spread on funds provided									77,437	36,416	21,744
Net interest income before provision for credit losses									605,650	553,817	530,908
(Reversal of) provision for credit losses									54,891	44,454	31,835
Noninterest income									110,287	110,089	95,556
Noninterest expense									228,627	193,161	171,805
Segment income (loss) before income taxes									432,419	426,291	422,824
Segment net income									309,926	251,834	248,474
Segment assets	23,761,469				21,431,472				23,761,469	21,431,472	19,128,510
Other
Segment Reporting Information
Interest income									121,541	115,910	96,322
Charge for funds used									(54,174)	(64,256)	(47,646)
Interest spread on funds used									67,367	51,654	48,676
Interest expense									(63,550)	(38,677)	(27,771)
Credit on funds provided									49,826	27,454	21,383
Interest spread on funds provided									(13,724)	(11,223)	(6,388)
Net interest income before provision for credit losses									53,643	40,431	42,288
(Reversal of) provision for credit losses									0	0	0
Noninterest income									15,015	93,208	35,471
Noninterest expense									149,427	148,645	137,058
Segment income (loss) before income taxes									(80,769)	(15,006)	(59,299)
Segment net income									59,520	63,386	60,947
Segment assets	$ 6,693,266				$ 6,373,504				$ 6,693,266	$ 6,373,504	$ 7,838,720