Loans, Changes in the Allowance for Loan Losses by Portfolio Segment (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2016	Dec. 31, 2015
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of year	$ 274,264	$ 266,302
Charge offs	(21,245)	(52,280)
Recoveries	4,351	22,242
Net charge offs	(16,894)	(30,038)
Provision for loan losses	20,000	38,000
Balance at end of year	277,370	274,264
Allowance for loan losses:
Individually evaluated for impairment			$ 14,093	$ 7,963
Collectively evaluated for impairment			263,277	266,301
Total allowance for loan losses	274,264	266,302	277,370	274,264
Loans [Abstract]
Individually evaluated for impairment			220,794	111,487
Collectively evaluated for impairment			19,006,446	18,602,856
Total loans			19,227,240	18,714,343
Commercial and industrial
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of year	129,959	117,635
Charge offs	(16,894)	(27,687)
Recoveries	1,958	9,821
Net charge offs	(14,936)	(17,866)
Provision for loan losses	24,470	30,190
Balance at end of year	139,493	129,959
Allowance for loan losses:
Individually evaluated for impairment			14,005	7,522
Collectively evaluated for impairment			125,488	122,437
Total allowance for loan losses	129,959	117,635	139,493	129,959
Loans [Abstract]
Individually evaluated for impairment			195,341	91,569
Collectively evaluated for impairment			6,316,307	6,099,114
Total loans			6,511,648	6,190,683
Commercial real estate — owner occupied
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of year	18,680	16,510
Charge offs	(76)	(2,645)
Recoveries	33	921
Net charge offs	(43)	(1,724)
Provision for loan losses	(2,059)	3,894
Balance at end of year	16,578	18,680
Allowance for loan losses:
Individually evaluated for impairment			74	0
Collectively evaluated for impairment			16,504	18,680
Total allowance for loan losses	18,680	16,510	16,578	18,680
Loans [Abstract]
Individually evaluated for impairment			7,500	6,221
Collectively evaluated for impairment			909,785	911,991
Total loans			917,285	918,212
Commercial real estate — investor
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of year	43,018	46,333
Charge offs	(176)	(4,645)
Recoveries	1,415	4,157
Net charge offs	1,239	(488)
Provision for loan losses	(2,233)	(2,827)
Balance at end of year	42,024	43,018
Allowance for loan losses:
Individually evaluated for impairment			0	229
Collectively evaluated for impairment			42,024	42,789
Total allowance for loan losses	43,018	46,333	42,024	43,018
Loans [Abstract]
Individually evaluated for impairment			9,829	5,460
Collectively evaluated for impairment			3,266,904	3,228,806
Total loans			3,276,733	3,234,266
Real Estate Construction Portfolio Segment [Member]
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of year	25,266	20,999
Charge offs	(54)	(750)
Recoveries	26	2,268
Net charge offs	(28)	1,518
Provision for loan losses	(3,498)	2,749
Balance at end of year	21,740	25,266
Allowance for loan losses:
Individually evaluated for impairment			0	0
Collectively evaluated for impairment			21,740	25,266
Total allowance for loan losses	25,266	20,999	21,740	25,266
Loans [Abstract]
Individually evaluated for impairment			0	0
Collectively evaluated for impairment			1,184,398	1,162,145
Total loans			1,184,398	1,162,145
Residential Portfolio Segment [Member]
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of year	28,261	31,926
Charge offs	(1,390)	(5,636)
Recoveries	158	1,077
Net charge offs	(1,232)	(4,559)
Provision for loan losses	2,030	894
Balance at end of year	29,059	28,261
Allowance for loan losses:
Individually evaluated for impairment			14	166
Collectively evaluated for impairment			29,045	28,095
Total allowance for loan losses	28,261	31,926	29,059	28,261
Loans [Abstract]
Individually evaluated for impairment			7,472	6,956
Collectively evaluated for impairment			5,936,985	5,776,311
Total loans			5,944,457	5,783,267
Home Equity Portfolio Segments [Member]
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of year	23,555	26,464
Charge offs	(1,746)	(7,048)
Recoveries	600	3,233
Net charge offs	(1,146)	(3,815)
Provision for loan losses	721	906
Balance at end of year	23,130	23,555
Allowance for loan losses:
Individually evaluated for impairment			0	46
Collectively evaluated for impairment			23,130	23,509
Total allowance for loan losses	23,555	26,464	23,130	23,555
Loans [Abstract]
Individually evaluated for impairment			652	1,281
Collectively evaluated for impairment			982,342	1,004,521
Total loans			982,994	1,005,802
Other consumer [Member]
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of year	5,525	6,435
Charge offs	(909)	(3,869)
Recoveries	161	765
Net charge offs	(748)	(3,104)
Provision for loan losses	569	2,194
Balance at end of year	5,346	5,525
Allowance for loan losses:
Individually evaluated for impairment			0	0
Collectively evaluated for impairment			5,346	5,525
Total allowance for loan losses	$ 5,525	$ 6,435	5,346	5,525
Loans [Abstract]
Individually evaluated for impairment			0	0
Collectively evaluated for impairment			409,725	419,968
Total loans			$ 409,725	$ 419,968