Business Segments (Operating Results and Other Key Financial Measures) (Details) - USD ($) $ 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
Segment Reporting Information
Interest income	$ 359,765	$ 339,910	$ 322,775	$ 302,669	$ 302,127	$ 280,317	$ 278,865	$ 276,172	$ 1,325,119	$ 1,137,481	$ 1,053,815
Charge for funds used									(533,777)	(360,465)	(317,143)
Interest spread on funds used									791,342	777,016	736,672
Interest expense	(40,064)	(36,755)	(32,684)	(30,547)	(29,425)	(26,169)	(25,281)	(23,968)	(140,050)	(104,843)	(103,376)
Credit on funds provided									533,777	360,465	317,143
Interest spread on funds provided									393,727	255,622	213,767
Net interest income before provision for credit losses	319,701	303,155	290,091	272,122	272,702	254,148	253,584	252,204	1,185,069	1,032,638	950,439
Provision for credit losses	15,517	12,996	10,685	7,068	10,461	9,525	6,053	1,440	46,266	27,479	14,217
Noninterest income	45,359	49,624	47,400	116,023	48,800	49,341	44,264	40,513	258,406	182,918	183,383
Noninterest expense	175,416	164,499	169,121	153,073	149,904	170,500	148,879	146,606	662,109	615,889	540,884
Segment income (loss) before income taxes	174,127	175,284	157,685	228,004	161,137	123,464	142,916	144,671	735,100	572,188	578,721
Segment income after income taxes	84,898	$ 132,660	$ 118,330	$ 169,736	110,734	$ 110,143	$ 103,284	$ 107,516	505,624	431,677	384,677
Segment assets	37,150,249				34,788,840				37,150,249	34,788,840	32,350,922
Retail Banking
Segment Reporting Information
Interest income									364,906	315,146	331,755
Charge for funds used									(142,619)	(95,970)	(86,769)
Interest spread on funds used									222,287	219,176	244,986
Interest expense									(76,770)	(60,180)	(53,088)
Credit on funds provided									445,304	300,446	261,117
Interest spread on funds provided									368,534	240,266	208,029
Net interest income before provision for credit losses									590,821	459,442	453,015
Provision for credit losses									1,812	(4,356)	(5,835)
Noninterest income									55,093	51,435	46,265
Noninterest expense									320,287	306,570	276,144
Segment income (loss) before income taxes									323,815	208,663	228,971
Segment income after income taxes									190,404	122,256	134,383
Segment assets	9,316,587				7,821,610				9,316,587	7,821,610	7,095,737
Commercial Banking
Segment Reporting Information
Interest income									844,303	726,013	654,966
Charge for funds used									(326,902)	(216,849)	(163,601)
Interest spread on funds used									517,401	509,164	491,365
Interest expense									(24,603)	(16,892)	(18,025)
Credit on funds provided									61,019	38,636	36,251
Interest spread on funds provided									36,416	21,744	18,226
Net interest income before provision for credit losses									553,817	530,908	509,591
Provision for credit losses									44,454	31,835	20,052
Noninterest income									110,104	96,010	71,867
Noninterest expense									193,176	172,259	159,987
Segment income (loss) before income taxes									426,291	422,824	401,419
Segment income after income taxes									251,834	248,474	236,459
Segment assets	21,431,472				19,128,510				21,431,472	19,128,510	17,923,319
Other
Segment Reporting Information
Interest income									115,910	96,322	67,094
Charge for funds used									(64,256)	(47,646)	(66,773)
Interest spread on funds used									51,654	48,676	321
Interest expense									(38,677)	(27,771)	(32,263)
Credit on funds provided									27,454	21,383	19,775
Interest spread on funds provided									(11,223)	(6,388)	(12,488)
Net interest income before provision for credit losses									40,431	42,288	(12,167)
Provision for credit losses									0	0	0
Noninterest income									93,209	35,473	65,251
Noninterest expense									148,646	137,060	104,753
Segment income (loss) before income taxes									(15,006)	(59,299)	(51,669)
Segment income after income taxes									63,386	60,947	13,835
Segment assets	$ 6,402,190				$ 7,838,720				$ 6,402,190	$ 7,838,720	$ 7,331,866