Loans and Allowance for Credit Losses, Impaired Loans (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2017		Jun. 30, 2016		Jun. 30, 2017		Jun. 30, 2016		Dec. 31, 2016
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	$ 480,181		$ 496,841		$ 480,181		$ 496,841		$ 473,526
Impaired loans, recorded investment, total	428,116		420,175		428,116		420,175		418,525
Impaired loans, recorded investment with no related allowance	355,469		388,031		355,469		388,031		407,363
Impaired loans, recorded investment with related allowance	72,647		32,144		72,647		32,144		11,162
Impaired loans, related allowance	9,693		4,310		9,693		4,310		843
Impaired loans, average recorded investment	419,893		416,291		432,921		355,154
Impaired loans, interest income recognized	2,328		2,327		4,523		4,426
Commercial
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	233,643		233,857		233,643		233,857		211,762
Impaired loans, recorded investment, total	197,157		181,989		197,157		181,989		178,953
Impaired loans, recorded investment with no related allowance	124,510		150,108		124,510		150,108		167,837
Impaired loans, recorded investment with related allowance	72,647		31,881		72,647		31,881		11,116
Impaired loans, related allowance	9,693		4,228		9,693		4,228		797
Impaired loans, average recorded investment	176,992		178,322		188,055		112,462
Impaired loans, interest income recognized	0		0		0		0
Commercial | Energy [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	141,091		202,369		141,091		202,369		146,897
Impaired loans, recorded investment, total	123,992		168,145		123,992		168,145		132,499
Impaired loans, recorded investment with no related allowance	56,988		136,264		56,988		136,264		121,418
Impaired loans, recorded investment with related allowance	67,004		31,881		67,004		31,881		11,081
Impaired loans, related allowance	8,874		4,228		8,874		4,228		762
Impaired loans, average recorded investment	117,209		163,849		128,246		97,923
Impaired loans, interest income recognized	0		0		0		0
Commercial | Services [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	11,209		12,780		11,209		12,780		11,723
Impaired loans, recorded investment, total	7,754		9,388		7,754		9,388		8,173
Impaired loans, recorded investment with no related allowance	7,754		9,388		7,754		9,388		8,173
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	7,734		9,450		7,964		9,839
Impaired loans, interest income recognized	0		0		0		0
Commercial | Wholesale/retail [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	17,392		8,697		17,392		8,697		17,669
Impaired loans, recorded investment, total	10,620		2,772		10,620		2,772		11,407
Impaired loans, recorded investment with no related allowance	10,620		2,772		10,620		2,772		11,407
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	10,855		3,229		11,013		2,846
Impaired loans, interest income recognized	0		0		0		0
Commercial | Manufacturing [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	10,223		650		10,223		650		5,320
Impaired loans, recorded investment, total	9,656		293		9,656		293		4,931
Impaired loans, recorded investment with no related allowance	9,656		293		9,656		293		4,931
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	7,781		303		7,293		312
Impaired loans, interest income recognized	0		0		0		0
Commercial | Healthcare [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	24,795		1,175		24,795		1,175		1,147
Impaired loans, recorded investment, total	24,505		875		24,505		875		825
Impaired loans, recorded investment with no related allowance	18,883		875		18,883		875		825
Impaired loans, recorded investment with related allowance	5,622		0		5,622		0		0
Impaired loans, related allowance	802		0		802		0		0
Impaired loans, average recorded investment	12,707		949		12,665		973
Impaired loans, interest income recognized	0		0		0		0
Commercial | Other commercial and industrial [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	28,933		8,186		28,933		8,186		29,006
Impaired loans, recorded investment, total	20,630		516		20,630		516		21,118
Impaired loans, recorded investment with no related allowance	20,609		516		20,609		516		21,083
Impaired loans, recorded investment with related allowance	21		0		21		0		35
Impaired loans, related allowance	17		0		17		0		35
Impaired loans, average recorded investment	20,706		542		20,874		569
Impaired loans, interest income recognized	0		0		0		0
Commercial real estate
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	6,905		18,519		6,905		18,519		14,427
Impaired loans, recorded investment, total	3,775		7,780		3,775		7,780		5,521
Impaired loans, recorded investment with no related allowance	3,775		7,628		3,775		7,628		5,521
Impaired loans, recorded investment with related allowance	0		152		0		152		0
Impaired loans, related allowance	0		18		0		18		0
Impaired loans, average recorded investment	4,125		8,524		4,648		8,390
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	3,676		7,177		3,676		7,177		4,951
Impaired loans, recorded investment, total	2,051		4,261		2,051		4,261		3,433
Impaired loans, recorded investment with no related allowance	2,051		4,261		2,051		4,261		3,433
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	2,334		4,525		2,742		4,335
Impaired loans, interest income recognized	0		0		0		0
Commercial real estate | Retail [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	518		1,914		518		1,914		530
Impaired loans, recorded investment, total	301		1,265		301		1,265		326
Impaired loans, recorded investment with no related allowance	301		1,265		301		1,265		326
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	308		1,283		314		1,292
Impaired loans, interest income recognized	0		0		0		0
Commercial real estate | Office [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	499		907		499		907		521
Impaired loans, recorded investment, total	396		606		396		606		426
Impaired loans, recorded investment with no related allowance	396		606		396		606		426
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	404		618		411		628
Impaired loans, interest income recognized	0		0		0		0
Commercial real estate | Multifamily [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	1,000		1,000		1,000		1,000		1,000
Impaired loans, recorded investment, total	10		65		10		65		38
Impaired loans, recorded investment with no related allowance	10		65		10		65		38
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	17		157		24		169
Impaired loans, interest income recognized	0		0		0		0
Commercial real estate | Industrial [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	0		76		0		76		76
Impaired loans, recorded investment, total	0		76		0		76		76
Impaired loans, recorded investment with no related allowance	0		76		0		76		76
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	38		76		38		76
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	1,212		7,445		1,212		7,445		7,349
Impaired loans, recorded investment, total	1,017		1,507		1,017		1,507		1,222
Impaired loans, recorded investment with no related allowance	1,017		1,355		1,017		1,355		1,222
Impaired loans, recorded investment with related allowance	0		152		0		152		0
Impaired loans, related allowance	0		18		0		18		0
Impaired loans, average recorded investment	1,024		1,865		1,119		1,890
Impaired loans, interest income recognized	0		0		0		0
Residential mortgage
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	239,326		243,291		239,326		243,291		247,005
Impaired loans, recorded investment, total	226,912		230,052		226,912		230,052		233,761
Impaired loans, recorded investment with no related allowance	226,912		229,941		226,912		229,941		233,715
Impaired loans, recorded investment with related allowance	0		111		0		111		46
Impaired loans, related allowance	0		64		0		64		46
Impaired loans, average recorded investment	238,523		229,103		239,937		233,893
Impaired loans, interest income recognized	2,328		2,327		4,523		4,426
Residential mortgage | Permanent mortgage [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	28,603		33,793		28,603		33,793		28,830
Impaired loans, recorded investment, total	23,415		27,228		23,415		27,228		22,855
Impaired loans, recorded investment with no related allowance	23,415		27,117		23,415		27,117		22,809
Impaired loans, recorded investment with related allowance	0		111		0		111		46
Impaired loans, related allowance	0		64		0		64		46
Impaired loans, average recorded investment	23,801		27,362		23,135		28,106
Impaired loans, interest income recognized	307		304		598		631
Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	197,659	[1]	198,534	[2]	197,659	[1]	198,534	[2]	205,564	[3]
Impaired loans, recorded investment, total	191,729	[1]	192,732	[2]	191,729	[1]	192,732	[2]	199,387	[3]
Impaired loans, recorded investment with no related allowance	191,729	[1]	192,732	[2]	191,729	[1]	192,732	[2]	199,387	[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	202,946	[1]	191,430	[2]	205,159	[1]	195,563	[2]
Impaired loans, interest income recognized	2,021	[1]	2,023	[2]	3,925	[1]	3,795	[2]
Residential mortgage | Home equity [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	13,064		10,964		13,064		10,964		12,611
Impaired loans, recorded investment, total	11,768		10,092		11,768		10,092		11,519
Impaired loans, recorded investment with no related allowance	11,768		10,092		11,768		10,092		11,519
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	11,776		10,311		11,643		10,224
Impaired loans, interest income recognized	0		0		0		0
Personal
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	307		1,174		307		1,174		332
Impaired loans, recorded investment, total	272		354		272		354		290
Impaired loans, recorded investment with no related allowance	272		354		272		354		290
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	253		342		281		409
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, 2017, $9.1 million of these loans were nonaccruing and $183 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, 2016, $20 million of these loans were nonaccruing and $173 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, 2016, $12 million of these loans were nonaccruing and $188 million were accruing based on the guarantee by U.S. government agencies.