Loans, Changes in the Allowance for Loan Losses by Portfolio Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of year	$ 278,335	$ 274,264
Charge offs	(56,246)	(86,439)
Recoveries	16,791	21,510
Net charge offs	(39,455)	(64,929)
Provision for loan losses	27,000	69,000
Balance at end of period	265,880	278,335
Allowance for loan losses:
Individually evaluated for impairment	25,846	35,054
Collectively evaluated for impairment	240,034	243,281
Balance at end of period	265,880	278,335
Loans:
Individually evaluated for impairment	247,575	316,735
Collectively evaluated for impairment	20,537,416	19,737,981
Loans and Leases Receivable, Net of Deferred Income	20,784,991	20,054,716
Commercial and industrial
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of year	140,126	129,959
Charge offs	(44,533)	(71,016)
Recoveries	11,465	14,543
Net charge offs	(33,068)	(56,473)
Provision for loan losses	16,010	66,640
Balance at end of period	123,068	140,126
Allowance for loan losses:
Individually evaluated for impairment	10,838	21,047
Collectively evaluated for impairment	112,230	119,079
Balance at end of period	123,068	140,126
Loans:
Individually evaluated for impairment	142,244	213,271
Collectively evaluated for impairment	6,257,449	6,275,743
Loans and Leases Receivable, Net of Deferred Income	6,399,693	6,489,014
Commercial real estate — owner occupied
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of year	14,034	18,680
Charge offs	(344)	(512)
Recoveries	173	74
Net charge offs	(171)	(438)
Provision for loan losses	(3,511)	(4,208)
Balance at end of period	10,352	14,034
Allowance for loan losses:
Individually evaluated for impairment	2,973	23
Collectively evaluated for impairment	7,379	14,011
Balance at end of period	10,352	14,034
Loans:
Individually evaluated for impairment	26,234	13,983
Collectively evaluated for impairment	775,975	883,741
Loans and Leases Receivable, Net of Deferred Income	802,209	897,724
Commercial real estate — investor
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of year	45,285	43,018
Charge offs	(991)	(1,504)
Recoveries	242	1,624
Net charge offs	(749)	120
Provision for loan losses	(3,477)	2,147
Balance at end of period	41,059	45,285
Allowance for loan losses:
Individually evaluated for impairment	1,597	3,410
Collectively evaluated for impairment	39,462	41,875
Balance at end of period	41,059	45,285
Loans:
Individually evaluated for impairment	19,118	32,908
Collectively evaluated for impairment	3,296,136	3,541,824
Loans and Leases Receivable, Net of Deferred Income	3,315,254	3,574,732
Real estate construction
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of year	26,932	25,266
Charge offs	(604)	(558)
Recoveries	74	203
Net charge offs	(530)	(355)
Provision for loan losses	7,968	2,021
Balance at end of period	34,370	26,932
Allowance for loan losses:
Individually evaluated for impairment	86	84
Collectively evaluated for impairment	34,284	26,848
Balance at end of period	34,370	26,932
Loans:
Individually evaluated for impairment	467	509
Collectively evaluated for impairment	1,451,217	1,431,988
Loans and Leases Receivable, Net of Deferred Income	1,451,684	1,432,497
Residential mortgage
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of year	27,046	28,261
Charge offs	(2,611)	(4,332)
Recoveries	927	755
Net charge offs	(1,684)	(3,577)
Provision for loan losses	4,245	2,362
Balance at end of period	29,607	27,046
Allowance for loan losses:
Individually evaluated for impairment	6,512	6,438
Collectively evaluated for impairment	23,095	20,608
Balance at end of period	29,607	27,046
Loans:
Individually evaluated for impairment	47,486	43,876
Collectively evaluated for impairment	7,499,048	6,288,451
Loans and Leases Receivable, Net of Deferred Income	7,546,534	6,332,327
Home equity
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of year	20,364	23,555
Charge offs	(2,724)	(4,686)
Recoveries	3,194	3,491
Net charge offs	470	(1,195)
Provision for loan losses	1,292	(1,996)
Balance at end of period	22,126	20,364
Allowance for loan losses:
Individually evaluated for impairment	3,718	3,943
Collectively evaluated for impairment	18,408	16,421
Balance at end of period	22,126	20,364
Loans:
Individually evaluated for impairment	10,891	11,176
Collectively evaluated for impairment	872,913	923,267
Loans and Leases Receivable, Net of Deferred Income	883,804	934,443
Other consumer
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of year	4,548	5,525
Charge offs	(4,439)	(3,831)
Recoveries	716	820
Net charge offs	(3,723)	(3,011)
Provision for loan losses	4,473	2,034
Balance at end of period	5,298	4,548
Allowance for loan losses:
Individually evaluated for impairment	122	109
Collectively evaluated for impairment	5,176	4,439
Balance at end of period	5,298	4,548
Loans:
Individually evaluated for impairment	1,135	1,012
Collectively evaluated for impairment	384,678	392,967
Loans and Leases Receivable, Net of Deferred Income	$ 385,813	$ 393,979