Loans and Allowance for Credit Losses, Impaired Loans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016		Mar. 31, 2015		Dec. 31, 2015
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	$ 485,090		$ 325,424		$ 369,450
Impaired loans, recorded investment, total	419,630		277,656		322,165
Impaired loans, recorded investment with no related allowance	384,749		276,400		278,245
Impaired loans, recorded investment with related allowance	34,881		1,256		43,920
Impaired loans, related allowance	2,686		317		16,393
Impaired loans, average recorded investment	356,708		285,063
Impaired loans, interest income recognized	2,099		2,571
Commercial [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	214,781		31,436		96,262
Impaired loans, recorded investment, total	174,652		13,880		76,424
Impaired loans, recorded investment with no related allowance	140,041		12,959		32,779
Impaired loans, recorded investment with related allowance	34,611		921		43,645
Impaired loans, related allowance	2,602		197		16,307
Impaired loans, average recorded investment	108,795		13,703
Impaired loans, interest income recognized	0		0
Commercial [Member] | Energy [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	182,224		1,884		63,910
Impaired loans, recorded investment, total	159,553		1,875		61,189
Impaired loans, recorded investment with no related allowance	125,072		1,875		18,330
Impaired loans, recorded investment with related allowance	34,481		0		42,859
Impaired loans, related allowance	2,558		0		16,115
Impaired loans, average recorded investment	93,627		1,646
Impaired loans, interest income recognized	0		0
Commercial [Member] | Services [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	12,824		7,698		13,449
Impaired loans, recorded investment, total	9,512		4,744		10,290
Impaired loans, recorded investment with no related allowance	9,512		4,051		9,657
Impaired loans, recorded investment with related allowance	0		693		633
Impaired loans, related allowance	0		153		148
Impaired loans, average recorded investment	9,901		4,972
Impaired loans, interest income recognized	0		0
Commercial [Member] | Wholesale/retail [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	9,502		9,953		8,582
Impaired loans, recorded investment, total	3,685		4,401		2,919
Impaired loans, recorded investment with no related allowance	3,673		4,369		2,907
Impaired loans, recorded investment with related allowance	12		32		12
Impaired loans, related allowance	9		9		9
Impaired loans, average recorded investment	3,302		4,275
Impaired loans, interest income recognized	0		0
Commercial [Member] | Manufacturing [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	658		716		665
Impaired loans, recorded investment, total	312		417		331
Impaired loans, recorded investment with no related allowance	312		417		331
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	322		433
Impaired loans, interest income recognized	0		0
Commercial [Member] | Healthcare [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	1,338		2,626		1,352
Impaired loans, recorded investment, total	1,023		1,558		1,072
Impaired loans, recorded investment with no related allowance	905		1,362		931
Impaired loans, recorded investment with related allowance	118		196		141
Impaired loans, related allowance	35		35		35
Impaired loans, average recorded investment	1,048		1,469
Impaired loans, interest income recognized	0		0
Commercial [Member] | Other commercial and industrial [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	8,235		8,559		8,304
Impaired loans, recorded investment, total	567		885		623
Impaired loans, recorded investment with no related allowance	567		885		623
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	595		908
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	20,080		34,233		21,454
Impaired loans, recorded investment, total	9,270		19,902		9,001
Impaired loans, recorded investment with no related allowance	9,114		19,732		8,842
Impaired loans, recorded investment with related allowance	156		170		159
Impaired loans, related allowance	18		18		18
Impaired loans, average recorded investment	9,136		19,230
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	7,621		14,367		8,963
Impaired loans, recorded investment, total	4,789		9,598		4,409
Impaired loans, recorded investment with no related allowance	4,789		9,598		4,409
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	4,599		7,449
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member] | Retail [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	1,923		5,376		1,923
Impaired loans, recorded investment, total	1,302		3,857		1,319
Impaired loans, recorded investment with no related allowance	1,302		3,857		1,319
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	1,311		3,892
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member] | Office [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	920		4,464		937
Impaired loans, recorded investment, total	629		2,410		651
Impaired loans, recorded investment with no related allowance	629		2,410		651
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	640		2,915
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member] | Multifamily [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	1,192		0		1,192
Impaired loans, recorded investment, total	250		0		274
Impaired loans, recorded investment with no related allowance	250		0		274
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	262		0
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member] | 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		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	8,348		9,950		8,363
Impaired loans, recorded investment, total	2,224		3,961		2,272
Impaired loans, recorded investment with no related allowance	2,068		3,791		2,113
Impaired loans, recorded investment with related allowance	156		170		159
Impaired loans, related allowance	18		18		18
Impaired loans, average recorded investment	2,248		4,936
Impaired loans, interest income recognized	0		0
Residential Mortgage [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	249,866		259,273		251,245
Impaired loans, recorded investment, total	235,377		243,410		236,277
Impaired loans, recorded investment with no related allowance	235,263		243,245		236,161
Impaired loans, recorded investment with related allowance	114		165		116
Impaired loans, related allowance	66		102		68
Impaired loans, average recorded investment	238,380		251,615
Impaired loans, interest income recognized	2,099		2,571
Residential Mortgage [Member] | Permanent mortgage [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	35,149		42,011		37,273
Impaired loans, recorded investment, total	27,497		33,365		28,984
Impaired loans, recorded investment with no related allowance	27,383		33,200		28,868
Impaired loans, recorded investment with related allowance	114		165		116
Impaired loans, related allowance	66		102		68
Impaired loans, average recorded investment	28,240		34,105
Impaired loans, interest income recognized	327		315
Residential Mortgage [Member] | Permanent mortgages guaranteed by US government agencies [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	203,396	[1]	207,133	[2]	202,984	[3]
Impaired loans, recorded investment, total	197,350	[1]	200,179	[2]	196,937	[3]
Impaired loans, recorded investment with no related allowance	197,350	[1]	200,179	[2]	196,937	[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	199,697	[1]	207,795	[2]
Impaired loans, interest income recognized	1,772	[1]	2,256	[2]
Residential Mortgage [Member] | Home equity {Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	11,321		10,129		10,988
Impaired loans, recorded investment, total	10,530		9,866		10,356
Impaired loans, recorded investment with no related allowance	10,530		9,866		10,356
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	10,443		9,715
Impaired loans, interest income recognized	0		0
Personal [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	363		482		489
Impaired loans, recorded investment, total	331		464		463
Impaired loans, recorded investment with no related allowance	331		464		463
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		$ 0
Impaired loans, average recorded investment	397		515
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, 2016, $19.6 million of these loans were nonaccruing and $178 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, 2015, $3.3 million of these loans were nonaccruing and $197 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, 2015, $21.9 million of these loans were nonaccruing and $175 million were accruing based on the guarantee by U.S. government agencies.