Loans and Allowances for Credit Losses, Activity in Allowance for Credit Losses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Allowance for loan losses [Roll Forward]
Beginning balance	$ 207,457	$ 230,682	$ 246,159
Provision for loan losses	44,205	9,944	510
Loans charged off	(50,977)	(43,583)	(25,599)
Recoveries	10,074	10,414	9,612
Ending balance	210,759	207,457	230,682
Accrual for off-balance sheet credit losses [Roll Forward]
Beginning balance	1,790	3,734	11,244
Provision for off-balance sheet credit risk	(205)	(1,944)	(7,510)
Ending balance	1,585	1,790	3,734
Total provision for credit losses	44,000	8,000	(7,000)
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment				$ 21,570,036	$ 21,493,483
Individually measured for impairment, recorded investment				180,951	163,247
Total				21,750,987	21,656,730
Related allowance [Abstract]
Collectively measured for impairment, related allowance				176,150	180,957
Individually measured for impairment, related allowance				17,414	8,732
Total measured for impairment, related allowance	210,759	207,457	230,682	210,759	207,457
Commercial [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	102,226	124,269	140,213
Provision for loan losses	57,125	12,521	(595)
Loans charged off	(43,185)	(37,880)	(19,810)
Recoveries	2,021	3,316	4,461
Ending balance	118,187	102,226	124,269
Accrual for off-balance sheet credit losses [Roll Forward]
Beginning balance	1,655	3,644	11,063
Provision for off-balance sheet credit risk	(221)	(1,989)	(7,419)
Ending balance	1,434	1,655	3,644
Total provision for credit losses	56,904	10,532	(8,014)
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment				13,916,234	13,536,237
Individually measured for impairment, recorded investment				115,416	99,841
Total				14,031,650	13,636,078
Related allowance [Abstract]
Collectively measured for impairment, related allowance				100,773	93,494
Individually measured for impairment, related allowance				17,414	8,732
Total measured for impairment, related allowance	118,187	102,226	140,213	118,187	102,226
Commercial Real Estate [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	60,026	56,621	50,749
Provision for loan losses	(12,046)	(147)	4,008
Loans charged off	(1,161)	0	(76)
Recoveries	4,986	3,552	1,940
Ending balance	51,805	60,026	56,621
Accrual for off-balance sheet credit losses [Roll Forward]
Beginning balance	52	45	123
Provision for off-balance sheet credit risk	55	7	(78)
Ending balance	107	52	45
Total provision for credit losses	(11,991)	(140)	3,930
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment				4,406,157	4,743,192
Individually measured for impairment, recorded investment				27,626	21,621
Total				4,433,783	4,764,813
Related allowance [Abstract]
Collectively measured for impairment, related allowance				51,805	60,026
Individually measured for impairment, related allowance				0	0
Total measured for impairment, related allowance	60,026	60,026	56,621	51,805	60,026
Residential Mortgage [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	17,964	18,451	18,224
Provision for loan losses	(3,838)	(1,156)	116
Loans charged off	(288)	(378)	(649)
Recoveries	562	1,047	760
Ending balance	14,400	17,964	18,451
Accrual for off-balance sheet credit losses [Roll Forward]
Beginning balance	52	43	50
Provision for off-balance sheet credit risk	(8)	9	(7)
Ending balance	44	52	43
Total provision for credit losses	(3,846)	(1,147)	109
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment				2,046,550	2,188,478
Individually measured for impairment, recorded investment				37,622	41,555
Total				2,084,172	2,230,033
Related allowance [Abstract]
Collectively measured for impairment, related allowance				14,400	17,964
Individually measured for impairment, related allowance				0	0
Total measured for impairment, related allowance	17,964	17,964	18,451	14,400	17,964
Personal [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	9,473	9,124	8,773
Provision for loan losses	3,537	3,175	2,964
Loans charged off	(6,343)	(5,325)	(5,064)
Recoveries	2,505	2,499	2,451
Ending balance	9,172	9,473	9,124
Accrual for off-balance sheet credit losses [Roll Forward]
Beginning balance	31	2	8
Provision for off-balance sheet credit risk	(31)	29	(6)
Ending balance	0	31	2
Total provision for credit losses	3,506	3,204	2,958
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment				1,201,095	1,025,576
Individually measured for impairment, recorded investment				287	230
Total				1,201,382	1,025,806
Related allowance [Abstract]
Collectively measured for impairment, related allowance				9,172	9,473
Individually measured for impairment, related allowance				0	0
Total measured for impairment, related allowance	9,172	9,473	8,773	9,172	9,473
Specific Allowance [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	189,689
Ending balance	193,564	189,689
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment				21,570,036	21,493,483
Individually measured for impairment, recorded investment				180,951	163,247
Total				21,750,987	21,656,730
Related allowance [Abstract]
Collectively measured for impairment, related allowance				176,150	180,957
Individually measured for impairment, related allowance				17,414	8,732
Total measured for impairment, related allowance	189,689	189,689		193,564	189,689
Nonspecific Allowance [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	17,768	22,217	28,200
Provision for loan losses	(573)	(4,449)	(5,983)
Loans charged off	0	0	0
Recoveries	0	0	0
Ending balance	17,195	17,768	22,217
Accrual for off-balance sheet credit losses [Roll Forward]
Beginning balance	0	0	0
Provision for off-balance sheet credit risk	0	0	0
Ending balance	0	0	0
Total provision for credit losses	(573)	(4,449)	(5,983)
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 measured for impairment, related allowance	$ 17,195	$ 22,217	$ 22,217	$ 17,195	$ 17,768