Loans and Allowance for Credit Losses, Impaired Loans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017		Mar. 31, 2016		Dec. 31, 2016
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	$ 451,653		$ 485,090		$ 473,526
Impaired loans, recorded investment, total	401,689		419,630		418,525
Impaired loans, recorded investment with no related allowance	349,526		384,749		407,363
Impaired loans, recorded investment with related allowance	52,163		34,881		11,162
Impaired loans, related allowance	3,497		2,686		843
Impaired loans, average recorded investment	415,718		356,708
Impaired loans, interest income recognized	2,195		2,099
Commercial
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	191,335		214,781		211,762
Impaired loans, recorded investment, total	156,825		174,652		178,953
Impaired loans, recorded investment with no related allowance	104,707		140,041		167,837
Impaired loans, recorded investment with related allowance	52,118		34,611		11,116
Impaired loans, related allowance	3,452		2,602		797
Impaired loans, average recorded investment	167,889		108,795
Impaired loans, interest income recognized	0		0
Commercial | Energy [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	125,579		182,224		146,897
Impaired loans, recorded investment, total	110,425		159,553		132,499
Impaired loans, recorded investment with no related allowance	58,324		125,072		121,418
Impaired loans, recorded investment with related allowance	52,101		34,481		11,081
Impaired loans, related allowance	3,435		2,558		762
Impaired loans, average recorded investment	121,462		93,627
Impaired loans, interest income recognized	0		0
Commercial | Services [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	11,542		12,824		11,723
Impaired loans, recorded investment, total	7,713		9,512		8,173
Impaired loans, recorded investment with no related allowance	7,713		9,512		8,173
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	7,943		9,901
Impaired loans, interest income recognized	0		0
Commercial | Wholesale/retail [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	17,582		9,502		17,669
Impaired loans, recorded investment, total	11,090		3,685		11,407
Impaired loans, recorded investment with no related allowance	11,090		3,673		11,407
Impaired loans, recorded investment with related allowance	0		12		0
Impaired loans, related allowance	0		9		0
Impaired loans, average recorded investment	11,248		3,302
Impaired loans, interest income recognized	0		0
Commercial | Manufacturing [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	6,377		658		5,320
Impaired loans, recorded investment, total	5,907		312		4,931
Impaired loans, recorded investment with no related allowance	5,907		312		4,931
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	5,419		322
Impaired loans, interest income recognized	0		0
Commercial | Healthcare [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	1,379		1,338		1,147
Impaired loans, recorded investment, total	909		1,023		825
Impaired loans, recorded investment with no related allowance	909		905		825
Impaired loans, recorded investment with related allowance	0		118		0
Impaired loans, related allowance	0		35		0
Impaired loans, average recorded investment	867		1,048
Impaired loans, interest income recognized	0		0
Commercial | Other commercial and industrial [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	28,876		8,235		29,006
Impaired loans, recorded investment, total	20,781		567		21,118
Impaired loans, recorded investment with no related allowance	20,764		567		21,083
Impaired loans, recorded investment with related allowance	17		0		35
Impaired loans, related allowance	17		0		35
Impaired loans, average recorded investment	20,950		595
Impaired loans, interest income recognized	0		0
Commercial real estate
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	7,453		20,080		14,427
Impaired loans, recorded investment, total	4,475		9,270		5,521
Impaired loans, recorded investment with no related allowance	4,475		9,114		5,521
Impaired loans, recorded investment with related allowance	0		156		0
Impaired loans, related allowance	0		18		0
Impaired loans, average recorded investment	4,998		9,136
Impaired loans, interest income recognized	0		0
Commercial real estate | Residential construction and land development [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	4,126		7,621		4,951
Impaired loans, recorded investment, total	2,616		4,789		3,433
Impaired loans, recorded investment with no related allowance	2,616		4,789		3,433
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	3,024		4,599
Impaired loans, interest income recognized	0		0
Commercial real estate | Retail [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	523		1,923		530
Impaired loans, recorded investment, total	314		1,302		326
Impaired loans, recorded investment with no related allowance	314		1,302		326
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	320		1,311
Impaired loans, interest income recognized	0		0
Commercial real estate | Office [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	515		920		521
Impaired loans, recorded investment, total	413		629		426
Impaired loans, recorded investment with no related allowance	413		629		426
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	420		640
Impaired loans, interest income recognized	0		0
Commercial real estate | Multifamily [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	1,000		1,192		1,000
Impaired loans, recorded investment, total	24		250		38
Impaired loans, recorded investment with no related allowance	24		250		38
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	31		262
Impaired loans, interest income recognized	0		0
Commercial real estate | Industrial [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	76		76		76
Impaired loans, recorded investment, total	76		76		76
Impaired loans, recorded investment with no related allowance	76		76		76
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	76		76
Impaired loans, interest income recognized	0		0
Commercial real estate | Other commercial real estate [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	1,213		8,348		7,349
Impaired loans, recorded investment, total	1,032		2,224		1,222
Impaired loans, recorded investment with no related allowance	1,032		2,068		1,222
Impaired loans, recorded investment with related allowance	0		156		0
Impaired loans, related allowance	0		18		0
Impaired loans, average recorded investment	1,127		2,248
Impaired loans, interest income recognized	0		0
Residential mortgage
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	252,600		249,866		247,005
Impaired loans, recorded investment, total	240,154		235,377		233,761
Impaired loans, recorded investment with no related allowance	240,109		235,263		233,715
Impaired loans, recorded investment with related allowance	45		114		46
Impaired loans, related allowance	45		66		46
Impaired loans, average recorded investment	242,569		238,380
Impaired loans, interest income recognized	2,195		2,099
Residential mortgage | Permanent mortgage [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	29,355		35,149		28,830
Impaired loans, recorded investment, total	24,188		27,497		22,855
Impaired loans, recorded investment with no related allowance	24,143		27,383		22,809
Impaired loans, recorded investment with related allowance	45		114		46
Impaired loans, related allowance	45		66		46
Impaired loans, average recorded investment	23,521		28,240
Impaired loans, interest income recognized	291		327
Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	210,237	[1]	203,396	[2]	205,564	[3]
Impaired loans, recorded investment, total	204,181	[1]	197,350	[2]	199,387	[3]
Impaired loans, recorded investment with no related allowance	204,181	[1]	197,350	[2]	199,387	[3]
Impaired loans, recorded investment with related allowance	0	[1]	0	[2]	0	[3]
Impaired loans, related allowance	0	[1]	0	[2]	0	[3]
Impaired loans, average recorded investment	207,396	[1]	199,697	[2]
Impaired loans, interest income recognized	1,904	[1]	1,772	[2]
Residential mortgage | Home equity [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	13,008		11,321		12,611
Impaired loans, recorded investment, total	11,785		10,530		11,519
Impaired loans, recorded investment with no related allowance	11,785		10,530		11,519
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	11,652		10,443
Impaired loans, interest income recognized	0		0
Personal
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	265		363		332
Impaired loans, recorded investment, total	235		331		290
Impaired loans, recorded investment with no related allowance	235		331		290
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		$ 0
Impaired loans, average recorded investment	262		397
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 March 31, 2017, $10 million of these loans were nonaccruing and $194 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 March 31, 2016, $20 million of these loans were nonaccruing and $178 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.