Loans and Allowances for Credit Losses, Impaired Loans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016		Dec. 31, 2015
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	$ 473,526		$ 369,450
Impaired loans, recorded investment, total	418,525		322,165
Impaired loans, recorded investment with no related allowance	407,363		278,245
Impaired loans, recorded investment with related allowance	11,162		43,920
Impaired loans, related allowance	843		16,393
Impaired loans, average recorded investment	349,330		297,971
Impaired loans, interest income recognized	9,014		9,056
Commercial [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	211,762		96,262
Impaired loans, recorded investment, total	178,953		76,424
Impaired loans, recorded investment with no related allowance	167,837		32,779
Impaired loans, recorded investment with related allowance	11,116		43,645
Impaired loans, related allowance	797		16,307
Impaired loans, average recorded investment	110,945		44,977
Impaired loans, interest income recognized	0		0
Commercial [Member] | Energy [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	146,897		63,910
Impaired loans, recorded investment, total	132,499		61,189
Impaired loans, recorded investment with no related allowance	121,418		18,330
Impaired loans, recorded investment with related allowance	11,081		42,859
Impaired loans, related allowance	762		16,115
Impaired loans, average recorded investment	80,100		31,303
Impaired loans, interest income recognized	0		0
Commercial [Member] | Services [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	11,723		13,449
Impaired loans, recorded investment, total	8,173		10,290
Impaired loans, recorded investment with no related allowance	8,173		9,657
Impaired loans, recorded investment with related allowance	0		633
Impaired loans, related allowance	0		148
Impaired loans, average recorded investment	9,232		7,746
Impaired loans, interest income recognized	0		0
Commercial [Member] | Wholesale/retail [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	17,669		8,582
Impaired loans, recorded investment, total	11,407		2,919
Impaired loans, recorded investment with no related allowance	11,407		2,907
Impaired loans, recorded investment with related allowance	0		12
Impaired loans, related allowance	0		9
Impaired loans, average recorded investment	7,163		3,534
Impaired loans, interest income recognized	0		0
Commercial [Member] | Manufacturing [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	5,320		665
Impaired loans, recorded investment, total	4,931		331
Impaired loans, recorded investment with no related allowance	4,931		331
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	2,631		391
Impaired loans, interest income recognized	0		0
Commercial [Member] | Healthcare [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	1,147		1,352
Impaired loans, recorded investment, total	825		1,072
Impaired loans, recorded investment with no related allowance	825		931
Impaired loans, recorded investment with related allowance	0		141
Impaired loans, related allowance	0		35
Impaired loans, average recorded investment	949		1,226
Impaired loans, interest income recognized	0		0
Commercial [Member] | Other commercial and industrial [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	29,006		8,304
Impaired loans, recorded investment, total	21,118		623
Impaired loans, recorded investment with no related allowance	21,083		623
Impaired loans, recorded investment with related allowance	35		0
Impaired loans, related allowance	35		0
Impaired loans, average recorded investment	10,870		777
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	14,427		21,454
Impaired loans, recorded investment, total	5,521		9,001
Impaired loans, recorded investment with no related allowance	5,521		8,842
Impaired loans, recorded investment with related allowance	0		159
Impaired loans, related allowance	0		18
Impaired loans, average recorded investment	7,262		13,778
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	4,951		8,963
Impaired loans, recorded investment, total	3,433		4,409
Impaired loans, recorded investment with no related allowance	3,433		4,409
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	3,921		4,854
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member] | Retail [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	530		1,923
Impaired loans, recorded investment, total	326		1,319
Impaired loans, recorded investment with no related allowance	326		1,319
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	823		2,622
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member] | Office [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	521		937
Impaired loans, recorded investment, total	426		651
Impaired loans, recorded investment with no related allowance	426		651
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	539		2,035
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member] | Multifamily [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	1,000		1,192
Impaired loans, recorded investment, total	38		274
Impaired loans, recorded investment with no related allowance	38		274
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	156		137
Impaired loans, interest income recognized	0		0
Commercial Real Estate [Member] | Industrial [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	76		76
Impaired loans, recorded investment, total	76		76
Impaired loans, recorded investment with no related allowance	76		76
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	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	7,349		8,363
Impaired loans, recorded investment, total	1,222		2,272
Impaired loans, recorded investment with no related allowance	1,222		2,113
Impaired loans, recorded investment with related allowance	0		159
Impaired loans, related allowance	0		18
Impaired loans, average recorded investment	1,747		4,092
Impaired loans, interest income recognized	0		0
Residential Mortgage [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	247,005		251,245
Impaired loans, recorded investment, total	233,761		236,277
Impaired loans, recorded investment with no related allowance	233,715		236,161
Impaired loans, recorded investment with related allowance	46		116
Impaired loans, related allowance	46		68
Impaired loans, average recorded investment	230,746		238,701
Impaired loans, interest income recognized	9,014		9,056
Residential Mortgage [Member] | Permanent mortgage [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	28,830		37,273
Impaired loans, recorded investment, total	22,855		28,984
Impaired loans, recorded investment with no related allowance	22,809		28,868
Impaired loans, recorded investment with related allowance	46		116
Impaired loans, related allowance	46		68
Impaired loans, average recorded investment	25,920		31,914
Impaired loans, interest income recognized	1,255		1,242
Residential Mortgage [Member] | Permanent mortgages guaranteed by U.S. government agencies [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	205,564	[1]	202,984	[2]
Impaired loans, recorded investment, total	199,387	[1]	196,937	[2]
Impaired loans, recorded investment with no related allowance	199,387	[1]	196,937	[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	193,889	[1]	196,827	[2]
Impaired loans, interest income recognized	7,759	[1]	7,814	[2]
Residential Mortgage [Member] | Home equity [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	12,611		10,988
Impaired loans, recorded investment, total	11,519		10,356
Impaired loans, recorded investment with no related allowance	11,519		10,356
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	10,937		9,960
Impaired loans, interest income recognized	0		0
Personal [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	332		489
Impaired loans, recorded investment, total	290		463
Impaired loans, recorded investment with no related allowance	290		463
Impaired loans, recorded investment with related allowance	0		0
Impaired loans, related allowance	0		0
Impaired loans, average recorded investment	377		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 December 31, 2016, $12 million of these loans are nonaccruing and $188 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, 2015, $22 million of these loans are nonaccruing and $175 million are accruing based on the guarantee by U.S. government agencies.