Loans and Allowances for Credit Losses, Activity in Allowance for Credit Losses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Dec. 31, 2018
Allowance for loan losses [Roll Forward]
Beginning balance	$ 205,340	$ 223,967	$ 207,457	$ 230,682
Provision for loan losses	4,918	1,702	12,887	(3,699)
Loans charged off	(13,227)	(15,105)	(25,002)	(17,995)
Recoveries	5,503	4,578	7,192	6,154
Ending balance	202,534	215,142	202,534	215,142
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	1,821	4,135	1,790	3,734
Provision for off-balance sheet credit losses	82	(1,702)	113	(1,301)
Ending balance	1,903	2,433	1,903	2,433
Total provision for credit losses	5,000	0	13,000	(5,000)
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment					$ 22,071,873	$ 21,493,483
Individually measured for impairment, recorded investment					183,779	163,247
Total					22,255,652	21,656,730
Related allowance [Abstract]
Collectively measured for impairment, related allowance					181,659	180,957
Individually measured for impairment, related allowance					4,038	8,732
Total	205,340	223,967	207,457	230,682	202,534	207,457
Commercial [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	103,577	120,083	102,226	124,269
Provision for loan losses	13,771	7,116	24,879	4,005
Loans charged off	(11,385)	(13,775)	(21,853)	(15,338)
Recoveries	434	298	1,145	786
Ending balance	106,397	113,722	106,397	113,722
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	1,725	4,027	1,655	3,644
Provision for off-balance sheet credit losses	17	(1,666)	87	(1,283)
Ending balance	1,742	2,361	1,742	2,361
Total provision for credit losses	13,788	5,450	24,966	2,722
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment					14,213,513	13,536,237
Individually measured for impairment, recorded investment					123,395	99,841
Total					14,336,908	13,636,078
Related allowance [Abstract]
Collectively measured for impairment, related allowance					102,359	93,494
Individually measured for impairment, related allowance					4,038	8,732
Total	103,577	120,083	106,397	113,722	106,397	102,226
Commercial real estate [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	58,134	57,070	60,026	56,621
Provision for loan losses	(8,173)	(1,409)	(10,177)	(1,143)
Loans charged off	(118)	0	(118)	0
Recoveries	4,345	3,097	4,457	3,280
Ending balance	54,188	58,758	54,188	58,758
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	48	44	52	45
Provision for off-balance sheet credit losses	68	(27)	64	(28)
Ending balance	116	17	116	17
Total provision for credit losses	(8,105)	(1,436)	(10,113)	(1,171)
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment					4,688,363	4,743,192
Individually measured for impairment, recorded investment					21,670	21,621
Total					4,710,033	4,764,813
Related allowance [Abstract]
Collectively measured for impairment, related allowance					54,188	60,026
Individually measured for impairment, related allowance					0	0
Total	58,134	57,070	54,188	56,621	54,188	60,026
Residential mortgage [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	15,668	18,431	17,964	18,451
Provision for loan losses	1	(257)	(2,407)	(419)
Loans charged off	(94)	(135)	(136)	(235)
Recoveries	149	505	303	747
Ending balance	15,724	18,544	15,724	18,544
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	47	62	52	43
Provision for off-balance sheet credit losses	(3)	(9)	(8)	10
Ending balance	44	53	44	53
Total provision for credit losses	(2)	(266)	(2,415)	(409)
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment					2,132,345	2,188,478
Individually measured for impairment, recorded investment					38,477	41,555
Total					2,170,822	2,230,033
Related allowance [Abstract]
Collectively measured for impairment, related allowance					15,724	17,964
Individually measured for impairment, related allowance					0	0
Total	15,668	18,431	17,964	18,451	15,724	17,964
Personal [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	8,783	8,408	9,473	9,124
Provision for loan losses	1,660	755	1,523	603
Loans charged off	(1,630)	(1,195)	(2,895)	(2,422)
Recoveries	575	678	1,287	1,341
Ending balance	9,388	8,646	9,388	8,646
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	1	2	31	2
Provision for off-balance sheet credit losses	0	0	(30)	0
Ending balance	1	2	1	2
Total provision for credit losses	1,660	755	1,493	603
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment					1,037,652	1,025,576
Individually measured for impairment, recorded investment					237	230
Total					1,037,889	1,025,806
Related allowance [Abstract]
Collectively measured for impairment, related allowance					9,388	9,473
Individually measured for impairment, related allowance					0	0
Total	8,783	8,408	9,388	9,124	9,388	9,473
Specific Allowance [Member]
Allowance for loan losses [Roll Forward]
Beginning balance			189,689
Ending balance	185,697		185,697
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment					22,071,873	21,493,483
Individually measured for impairment, recorded investment					183,779	163,247
Total					22,255,652	21,656,730
Related allowance [Abstract]
Collectively measured for impairment, related allowance					181,659	180,957
Individually measured for impairment, related allowance					4,038	8,732
Total	185,697		189,689		185,697	189,689
Nonspecific Allowance [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	19,178	19,975	17,768	22,217
Provision for loan losses	(2,341)	(4,503)	(931)	(6,745)
Loans charged off	0	0	0	0
Recoveries	0	0	0	0
Ending balance	16,837	15,472	16,837	15,472
Allowance for off-balance sheet credit losses [Roll Forward]
Beginning balance	0	0	0	0
Provision for off-balance sheet credit losses	0	0	0	0
Ending balance	0	0	0	0
Total provision for credit losses	(2,341)	(4,503)	(931)	(6,745)
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment					0	0
Individually measured for impairment, recorded investment					0	0
Total					0	0
Related allowance [Abstract]
Collectively measured for impairment, related allowance					0	0
Individually measured for impairment, related allowance					0	0
Total	$ 19,178	$ 19,975	$ 16,837	$ 15,472	$ 16,837	$ 17,768