Loans and Allowances for Credit Losses, Impaired Loans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019		Dec. 31, 2018
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	$ 467,714		$ 423,787
Impaired loans, recorded investment, total	372,645		346,981
Impaired loans, recorded investment with no related allowance	323,505		311,526
Impaired loans, recorded investment with related allowance	49,140		35,455
Impaired loans, related allowance	17,414		8,732
Impaired loans, average recorded investment	354,262		345,107
Impaired loans, interest income recognized	8,931		8,405
Commercial [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	195,193		156,163
Impaired loans, recorded investment, total	115,416		99,841
Impaired loans, recorded investment with no related allowance	66,276		64,386
Impaired loans, recorded investment with related allowance	49,140		35,455
Impaired loans, related allowance	17,414		8,732
Impaired loans, average recorded investment	101,399		115,460
Impaired loans, interest income recognized	0		0
Commercial [Member] | Energy [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	149,441		79,675
Impaired loans, recorded investment, total	91,722		47,494
Impaired loans, recorded investment with no related allowance	44,244		18,639
Impaired loans, recorded investment with related allowance	47,478		28,855
Impaired loans, related allowance	16,854		5,362
Impaired loans, average recorded investment	69,119		69,645
Impaired loans, interest income recognized	0		0
Commercial [Member] | Services [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	10,923		13,437
Impaired loans, recorded investment, total	7,483		8,567
Impaired loans, recorded investment with no related allowance	6,301		8,489
Impaired loans, recorded investment with related allowance	1,182		78
Impaired loans, related allowance	240		74
Impaired loans, average recorded investment	5,854		4,509
Impaired loans, interest income recognized	0		0
Commercial [Member] | Wholesale/retail [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	1,980		1,722
Impaired loans, recorded investment, total	1,163		1,316
Impaired loans, recorded investment with no related allowance	902		1,015
Impaired loans, recorded investment with related allowance	261		301
Impaired loans, related allowance	101		101
Impaired loans, average recorded investment	916		1,784
Impaired loans, interest income recognized	0		0
Commercial [Member] | Manufacturing [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	10,848		10,055
Impaired loans, recorded investment, total	10,133		8,919
Impaired loans, recorded investment with no related allowance	9,914		8,673
Impaired loans, recorded investment with related allowance	219		246
Impaired loans, related allowance	219		246
Impaired loans, average recorded investment	9,144		7,249
Impaired loans, interest income recognized	0		0
Commercial [Member] | Healthcare [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	13,774		24,319
Impaired loans, recorded investment, total	4,480		16,538
Impaired loans, recorded investment with no related allowance	4,480		10,563
Impaired loans, recorded investment with related allowance	0		5,975
Impaired loans, related allowance	0		2,949
Impaired loans, average recorded investment	7,798		14,297
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	8,227		26,955
Impaired loans, recorded investment, total	435		17,007
Impaired loans, recorded investment with no related allowance	435		17,007
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	8,568		17,976
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	30,139		30,138
Impaired loans, recorded investment, total	27,626		21,621
Impaired loans, recorded investment with no related allowance	27,626		21,621
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	24,623		12,238
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		1,306
Impaired loans, recorded investment, total	350		350
Impaired loans, recorded investment with no related allowance	350		350
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	350		1,091
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member] | Retail [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	20,265		27,680
Impaired loans, recorded investment, total	18,868		20,279
Impaired loans, recorded investment with no related allowance	18,868		20,279
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	19,573		10,278
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member] | Office [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		137
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member] | Multifamily [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	6,858		301
Impaired loans, recorded investment, total	6,858		301
Impaired loans, recorded investment with no related allowance	6,858		301
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	3,580		151
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member] | Industrial [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	909		0
Impaired loans, recorded investment, total	909		0
Impaired loans, recorded investment with no related allowance	909		0
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	454		0
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member] | Other commercial real estate [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	801		851
Impaired loans, recorded investment, total	641		691
Impaired loans, recorded investment with no related allowance	641		691
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	666		581
Impaired loans, interest income recognized	0		0
Residential Mortgage [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	242,022		237,208
Impaired loans, recorded investment, total	229,316		225,289
Impaired loans, recorded investment with no related allowance	229,316		225,289
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	227,981		217,159
Impaired loans, interest income recognized	8,931		8,405
Residential Mortgage [Member] | Permanent mortgage [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	24,868		28,716
Impaired loans, recorded investment, total	20,441		23,951
Impaired loans, recorded investment with no related allowance	20,441		23,951
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	22,196		24,572
Impaired loans, interest income recognized	1,198		1,233
Residential Mortgage [Member] | Permanent mortgages guaranteed by U.S. government agencies [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	204,187	[1]	196,296	[2]
Impaired loans, recorded investment, total	197,794	[1]	190,866	[2]
Impaired loans, recorded investment with no related allowance	197,794	[1]	190,866	[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	195,009	[1]	180,813	[2]
Impaired loans, interest income recognized	7,733	[1]	7,172	[2]
Residential Mortgage [Member] | Home equity [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	12,967		12,196
Impaired loans, recorded investment, total	11,081		10,472
Impaired loans, recorded investment with no related allowance	11,081		10,472
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	10,776		11,774
Impaired loans, interest income recognized	0		0
Personal [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	360		278
Impaired loans, recorded investment, total	287		230
Impaired loans, recorded investment with no related allowance	287		230
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	259		250
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, 2019, $6.1 million of these loans are nonaccruing and $192 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, 2018, $7.1 million of these loans are nonaccruing and $184 million are accruing based on the guarantee by U.S. government agencies.