Loans and Allowance for Credit Losses, Impaired Loans (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2018		Jun. 30, 2017		Jun. 30, 2018		Jun. 30, 2017		Dec. 31, 2017
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	$ 389,643		$ 480,181		$ 389,643		$ 480,181		$ 438,459
Impaired loans, recorded investment, total	327,743		428,116		327,743		428,116		376,201
Impaired loans, recorded investment with no related allowance	267,728		355,469		267,728		355,469		324,867
Impaired loans, recorded investment with related allowance	60,015		72,647		60,015		72,647		51,334
Impaired loans, related allowance	15,200		9,693		15,200		9,693		8,831
Impaired loans, average recorded investment	335,492		419,893		349,062		432,921
Impaired loans, interest income recognized	1,896		2,328		4,050		4,523
Commercial
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	162,209		233,643		162,209		233,643		184,604
Impaired loans, recorded investment, total	120,978		197,157		120,978		197,157		137,303
Impaired loans, recorded investment with no related allowance	60,963		124,510		60,963		124,510		85,969
Impaired loans, recorded investment with related allowance	60,015		72,647		60,015		72,647		51,334
Impaired loans, related allowance	15,200		9,693		15,200		9,693		8,831
Impaired loans, average recorded investment	126,219		176,992		129,139		188,055
Impaired loans, interest income recognized	0		0		0		0
Commercial | Energy [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	84,285		141,091		84,285		141,091		111,011
Impaired loans, recorded investment, total	65,597		123,992		65,597		123,992		92,284
Impaired loans, recorded investment with no related allowance	19,735		56,988		19,735		56,988		40,968
Impaired loans, recorded investment with related allowance	45,862		67,004		45,862		67,004		51,316
Impaired loans, related allowance	9,460		8,874		9,460		8,874		8,814
Impaired loans, average recorded investment	77,770		117,209		78,940		128,246
Impaired loans, interest income recognized	0		0		0		0
Commercial | Services [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	7,211		11,209		7,211		11,209		5,324
Impaired loans, recorded investment, total	4,377		7,754		4,377		7,754		2,620
Impaired loans, recorded investment with no related allowance	4,296		7,754		4,296		7,754		2,620
Impaired loans, recorded investment with related allowance	81		0		81		0		0
Impaired loans, related allowance	79		0		79		0		0
Impaired loans, average recorded investment	3,243		7,734		3,498		7,964
Impaired loans, interest income recognized	0		0		0		0
Commercial | Wholesale/retail [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	14,523		17,392		14,523		17,392		9,099
Impaired loans, recorded investment, total	14,095		10,620		14,095		10,620		2,574
Impaired loans, recorded investment with no related allowance	2,822		10,620		2,822		10,620		2,574
Impaired loans, recorded investment with related allowance	11,273		0		11,273		0		0
Impaired loans, related allowance	4,075		0		4,075		0		0
Impaired loans, average recorded investment	8,329		10,855		8,334		11,013
Impaired loans, interest income recognized	0		0		0		0
Commercial | Manufacturing [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	2,995		10,223		2,995		10,223		6,073
Impaired loans, recorded investment, total	2,991		9,656		2,991		9,656		5,962
Impaired loans, recorded investment with no related allowance	2,734		9,656		2,734		9,656		5,962
Impaired loans, recorded investment with related allowance	257		0		257		0		0
Impaired loans, related allowance	257		0		257		0		0
Impaired loans, average recorded investment	2,996		7,781		4,476		7,293
Impaired loans, interest income recognized	0		0		0		0
Commercial | Healthcare [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	26,212		24,795		26,212		24,795		25,140
Impaired loans, recorded investment, total	16,125		24,505		16,125		24,505		14,765
Impaired loans, recorded investment with no related allowance	13,583		18,883		13,583		18,883		14,765
Impaired loans, recorded investment with related allowance	2,542		5,622		2,542		5,622		0
Impaired loans, related allowance	1,329		802		1,329		802		0
Impaired loans, average recorded investment	15,734		12,707		15,445		12,665
Impaired loans, interest income recognized	0		0		0		0
Commercial | Other commercial and industrial [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	26,983		28,933		26,983		28,933		27,957
Impaired loans, recorded investment, total	17,793		20,630		17,793		20,630		19,098
Impaired loans, recorded investment with no related allowance	17,793		20,609		17,793		20,609		19,080
Impaired loans, recorded investment with related allowance	0		21		0		21		18
Impaired loans, related allowance	0		17		0		17		17
Impaired loans, average recorded investment	18,147		20,706		18,446		20,874
Impaired loans, interest income recognized	0		0		0		0
Commercial real estate
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	10,694		6,905		10,694		6,905		4,751
Impaired loans, recorded investment, total	1,996		3,775		1,996		3,775		2,855
Impaired loans, recorded investment with no related allowance	1,996		3,775		1,996		3,775		2,855
Impaired loans, recorded investment with related allowance	0		0		0		0		0
Impaired loans, related allowance	0		0		0		0		0
Impaired loans, average recorded investment	2,234		4,125		2,425		4,648
Impaired loans, interest income recognized	0		0		0		0
Commercial real estate | Residential construction and land development [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	1,764		3,676		1,764		3,676		3,285
Impaired loans, recorded investment, total	350		2,051		350		2,051		1,832
Impaired loans, recorded investment with no related allowance	350		2,051		350		2,051		1,832
Impaired loans, recorded investment with related allowance	0		0		0		0		0
Impaired loans, related allowance	0		0		0		0		0
Impaired loans, average recorded investment	982		2,334		1,091		2,742
Impaired loans, interest income recognized	0		0		0		0
Commercial real estate | Retail [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	8,134		518		8,134		518		509
Impaired loans, recorded investment, total	1,068		301		1,068		301		276
Impaired loans, recorded investment with no related allowance	1,068		301		1,068		301		276
Impaired loans, recorded investment with related allowance	0		0		0		0		0
Impaired loans, related allowance	0		0		0		0		0
Impaired loans, average recorded investment	666		308		672		314
Impaired loans, interest income recognized	0		0		0		0
Commercial real estate | Office [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	287		499		287		499		287
Impaired loans, recorded investment, total	275		396		275		396		275
Impaired loans, recorded investment with no related allowance	275		396		275		396		275
Impaired loans, recorded investment with related allowance	0		0		0		0		0
Impaired loans, related allowance	0		0		0		0		0
Impaired loans, average recorded investment	275		404		275		411
Impaired loans, interest income recognized	0		0		0		0
Commercial real estate | Multifamily [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	0		1,000		0		1,000		0
Impaired loans, recorded investment, total	0		10		0		10		0
Impaired loans, recorded investment with no related allowance	0		10		0		10		0
Impaired loans, recorded investment with related allowance	0		0		0		0		0
Impaired loans, related allowance	0		0		0		0		0
Impaired loans, average recorded investment	0		17		0		24
Impaired loans, interest income recognized	0		0		0		0
Commercial real estate | Industrial [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	0		0		0		0		0
Impaired loans, recorded investment, total	0		0		0		0		0
Impaired loans, recorded investment with no related allowance	0		0		0		0		0
Impaired loans, recorded investment with related allowance	0		0		0		0		0
Impaired loans, related allowance	0		0		0		0		0
Impaired loans, average recorded investment	0		38		0		38
Impaired loans, interest income recognized	0		0		0		0
Commercial real estate | Other commercial real estate [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	509		1,212		509		1,212		670
Impaired loans, recorded investment, total	303		1,017		303		1,017		472
Impaired loans, recorded investment with no related allowance	303		1,017		303		1,017		472
Impaired loans, recorded investment with related allowance	0		0		0		0		0
Impaired loans, related allowance	0		0		0		0		0
Impaired loans, average recorded investment	311		1,024		387		1,119
Impaired loans, interest income recognized	0		0		0		0
Residential mortgage
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	216,353		239,326		216,353		239,326		248,797
Impaired loans, recorded investment, total	204,429		226,912		204,429		226,912		235,774
Impaired loans, recorded investment with no related allowance	204,429		226,912		204,429		226,912		235,774
Impaired loans, recorded investment with related allowance	0		0		0		0		0
Impaired loans, related allowance	0		0		0		0		0
Impaired loans, average recorded investment	206,699		238,523		217,193		239,937
Impaired loans, interest income recognized	1,896		2,328		4,050		4,523
Residential mortgage | Permanent mortgage [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	28,402		28,603		28,402		28,603		30,435
Impaired loans, recorded investment, total	23,105		23,415		23,105		23,415		25,193
Impaired loans, recorded investment with no related allowance	23,105		23,415		23,105		23,415		25,193
Impaired loans, recorded investment with related allowance	0		0		0		0		0
Impaired loans, related allowance	0		0		0		0		0
Impaired loans, average recorded investment	23,841		23,801		24,149		23,135
Impaired loans, interest income recognized	322		307		628		598
Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	174,589	[1]	197,659	[2]	174,589	[1]	197,659	[2]	203,814	[3]
Impaired loans, recorded investment, total	169,653	[1]	191,729	[2]	169,653	[1]	191,729	[2]	197,506	[3]
Impaired loans, recorded investment with no related allowance	169,653	[1]	191,729	[2]	169,653	[1]	191,729	[2]	197,506	[3]
Impaired loans, recorded investment with related allowance	0	[1]	0	[2]	0	[1]	0	[2]	0	[3]
Impaired loans, related allowance	0	[1]	0	[2]	0	[1]	0	[2]	0	[3]
Impaired loans, average recorded investment	170,856	[1]	202,946	[2]	180,671	[1]	205,159	[2]
Impaired loans, interest income recognized	1,574	[1]	2,021	[2]	3,422	[1]	3,925	[2]
Residential mortgage | Home equity [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	13,362		13,064		13,362		13,064		14,548
Impaired loans, recorded investment, total	11,671		11,768		11,671		11,768		13,075
Impaired loans, recorded investment with no related allowance	11,671		11,768		11,671		11,768		13,075
Impaired loans, recorded investment with related allowance	0		0		0		0		0
Impaired loans, related allowance	0		0		0		0		0
Impaired loans, average recorded investment	12,002		11,776		12,373		11,643
Impaired loans, interest income recognized	0		0		0		0
Personal
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	387		307		387		307		307
Impaired loans, recorded investment, total	340		272		340		272		269
Impaired loans, recorded investment with no related allowance	340		272		340		272		269
Impaired loans, recorded investment with related allowance	0		0		0		0		0
Impaired loans, related allowance	0		0		0		0		$ 0
Impaired loans, average recorded investment	340		253		305		281
Impaired loans, interest income recognized	$ 0		$ 0		$ 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 June 30, 2018, $7.6 million of these loans were nonaccruing and $162 million were 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 June 30, 2017, $9.1 million of these loans were nonaccruing and $183 million were accruing based on the guarantee by U.S. government agencies.

[3] 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 were nonaccruing and $188 million were accruing based on the guarantee by U.S. government agencies.