Loans and Allowances for Credit Losses, Impaired Loans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018		Dec. 31, 2017
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	$ 423,787		$ 438,459
Impaired loans, recorded investment, total	346,981		376,201
Impaired loans, recorded investment with no related allowance	311,526		324,867
Impaired loans, recorded investment with related allowance	35,455		51,334
Impaired loans, related allowance	8,732		8,831
Impaired loans, average recorded investment	345,107		398,161
Impaired loans, interest income recognized	8,405		8,861
Commercial [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	156,163		184,604
Impaired loans, recorded investment, total	99,841		137,303
Impaired loans, recorded investment with no related allowance	64,386		85,969
Impaired loans, recorded investment with related allowance	35,455		51,334
Impaired loans, related allowance	8,732		8,831
Impaired loans, average recorded investment	115,460		158,127
Impaired loans, interest income recognized	0		0
Commercial [Member] | Energy [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	79,675		111,011
Impaired loans, recorded investment, total	47,494		92,284
Impaired loans, recorded investment with no related allowance	18,639		40,968
Impaired loans, recorded investment with related allowance	28,855		51,316
Impaired loans, related allowance	5,362		8,814
Impaired loans, average recorded investment	69,645		112,392
Impaired loans, interest income recognized	0		0
Commercial [Member] | Services [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	13,437		5,324
Impaired loans, recorded investment, total	8,567		2,620
Impaired loans, recorded investment with no related allowance	8,489		2,620
Impaired loans, recorded investment with related allowance	78		0
Impaired loans, related allowance	74		0
Impaired loans, average recorded investment	4,509		5,396
Impaired loans, interest income recognized	0		0
Commercial [Member] | Wholesale/retail [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	1,722		9,099
Impaired loans, recorded investment, total	1,316		2,574
Impaired loans, recorded investment with no related allowance	1,015		2,574
Impaired loans, recorded investment with related allowance	301		0
Impaired loans, related allowance	101		0
Impaired loans, average recorded investment	1,784		6,990
Impaired loans, interest income recognized	0		0
Commercial [Member] | Manufacturing [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	10,055		6,073
Impaired loans, recorded investment, total	8,919		5,962
Impaired loans, recorded investment with no related allowance	8,673		5,962
Impaired loans, recorded investment with related allowance	246		0
Impaired loans, related allowance	246		0
Impaired loans, average recorded investment	7,249		5,446
Impaired loans, interest income recognized	0		0
Commercial [Member] | Healthcare [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	24,319		25,140
Impaired loans, recorded investment, total	16,538		14,765
Impaired loans, recorded investment with no related allowance	10,563		14,765
Impaired loans, recorded investment with related allowance	5,975		0
Impaired loans, related allowance	2,949		0
Impaired loans, average recorded investment	14,297		7,795
Impaired loans, interest income recognized	0		0
Commercial [Member] | Public finance [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	0		0
Impaired loans, recorded investment, total	0		0
Impaired loans, recorded investment with no related allowance	0		0
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	0		0
Impaired loans, interest income recognized	0		0
Commercial [Member] | Other commercial and industrial [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	26,955		27,957
Impaired loans, recorded investment, total	17,007		19,098
Impaired loans, recorded investment with no related allowance	17,007		19,080
Impaired loans, recorded investment with related allowance	0		18
Impaired loans, related allowance	0		17
Impaired loans, average recorded investment	17,976		20,108
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	30,138		4,751
Impaired loans, recorded investment, total	21,621		2,855
Impaired loans, recorded investment with no related allowance	21,621		2,855
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	12,238		4,189
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member] | Residential construction and land development [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	1,306		3,285
Impaired loans, recorded investment, total	350		1,832
Impaired loans, recorded investment with no related allowance	350		1,832
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	1,091		2,633
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member] | Retail [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	27,680		509
Impaired loans, recorded investment, total	20,279		276
Impaired loans, recorded investment with no related allowance	20,279		276
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	10,278		301
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member] | Office [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	0		287
Impaired loans, recorded investment, total	0		275
Impaired loans, recorded investment with no related allowance	0		275
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	137		351
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member] | Multifamily [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	301		0
Impaired loans, recorded investment, total	301		0
Impaired loans, recorded investment with no related allowance	301		0
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	151		19
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member] | Industrial [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	0		0
Impaired loans, recorded investment, total	0		0
Impaired loans, recorded investment with no related allowance	0		0
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	0		38
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member] | Other commercial real estate [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	851		670
Impaired loans, recorded investment, total	691		472
Impaired loans, recorded investment with no related allowance	691		472
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	581		847
Impaired loans, interest income recognized	0		0
Residential Mortgage [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	237,208		248,797
Impaired loans, recorded investment, total	225,289		235,774
Impaired loans, recorded investment with no related allowance	225,289		235,774
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	217,159		235,565
Impaired loans, interest income recognized	8,405		8,861
Residential Mortgage [Member] | Permanent mortgage [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	28,716		30,435
Impaired loans, recorded investment, total	23,951		25,193
Impaired loans, recorded investment with no related allowance	23,951		25,193
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	24,572		24,024
Impaired loans, interest income recognized	1,233		1,229
Residential Mortgage [Member] | Permanent mortgages guaranteed by U.S. government agencies [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	196,296	[1]	203,814	[2]
Impaired loans, recorded investment, total	190,866	[1]	197,506	[2]
Impaired loans, recorded investment with no related allowance	190,866	[1]	197,506	[2]
Impaired loans, recorded investment with related allowance	0	[1]	0	[2]
Impaired loans, related allowance	0	[1]	0	[2]
Impaired loans, average recorded investment	180,813	[1]	199,244	[2]
Impaired loans, interest income recognized	7,172	[1]	7,632	[2]
Residential Mortgage [Member] | Home equity [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	12,196		14,548
Impaired loans, recorded investment, total	10,472		13,075
Impaired loans, recorded investment with no related allowance	10,472		13,075
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	11,774		12,297
Impaired loans, interest income recognized	0		0
Personal [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	278		307
Impaired loans, recorded investment, total	230		269
Impaired loans, recorded investment with no related allowance	230		269
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	250		280
Impaired loans, interest income recognized	$ 0		$ 0

[1] All permanent mortgage loans guaranteed by U.S. government agencies are considered impaired as we do not expect full collection of contractual principal and interest. At December 31, 2018, $7.1 million of these loans are nonaccruing and $184 million are accruing based on the guarantee by U.S. government agencies.

[2] All permanent mortgage loans guaranteed by U.S. government agencies are considered impaired as we do not expect full collection of contractual principal and interest. At December 31, 2017, $9.2 million of these loans are nonaccruing and $188 million are accruing based on the guarantee by U.S. government agencies.