Loans and Allowance for Credit Losses, Impaired Loans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018		Mar. 31, 2017		Dec. 31, 2017
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	$ 413,314		$ 451,653		$ 438,459
Impaired loans, recorded investment, total	349,061		401,689		376,201
Impaired loans, recorded investment with no related allowance	275,439		349,526		324,867
Impaired loans, recorded investment with related allowance	73,622		52,163		51,334
Impaired loans, related allowance	13,362		3,497		8,831
Impaired loans, average recorded investment	374,317		415,718
Impaired loans, interest income recognized	2,154		2,195
Commercial
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	181,507		191,335		184,604
Impaired loans, recorded investment, total	131,460		156,825		137,303
Impaired loans, recorded investment with no related allowance	57,838		104,707		85,969
Impaired loans, recorded investment with related allowance	73,622		52,118		51,334
Impaired loans, related allowance	13,362		3,452		8,831
Impaired loans, average recorded investment	134,381		167,889
Impaired loans, interest income recognized	0		0
Commercial | Energy [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	111,002		125,579		111,011
Impaired loans, recorded investment, total	89,942		110,425		92,284
Impaired loans, recorded investment with no related allowance	22,816		58,324		40,968
Impaired loans, recorded investment with related allowance	67,126		52,101		51,316
Impaired loans, related allowance	12,701		3,435		8,814
Impaired loans, average recorded investment	91,113		121,462
Impaired loans, interest income recognized	0		0
Commercial | Services [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	4,865		11,542		5,324
Impaired loans, recorded investment, total	2,109		7,713		2,620
Impaired loans, recorded investment with no related allowance	2,109		7,713		2,620
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	2,365		7,943
Impaired loans, interest income recognized	0		0
Commercial | Wholesale/retail [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	9,089		17,582		9,099
Impaired loans, recorded investment, total	2,564		11,090		2,574
Impaired loans, recorded investment with no related allowance	2,564		11,090		2,574
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	2,569		11,248
Impaired loans, interest income recognized	0		0
Commercial | Manufacturing [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	3,111		6,377		6,073
Impaired loans, recorded investment, total	3,002		5,907		5,962
Impaired loans, recorded investment with no related allowance	2,741		5,907		5,962
Impaired loans, recorded investment with related allowance	261		0		0
Impaired loans, related allowance	261		0		0
Impaired loans, average recorded investment	4,482		5,419
Impaired loans, interest income recognized	0		0
Commercial | Healthcare [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	26,019		1,379		25,140
Impaired loans, recorded investment, total	15,342		909		14,765
Impaired loans, recorded investment with no related allowance	9,107		909		14,765
Impaired loans, recorded investment with related allowance	6,235		0		0
Impaired loans, related allowance	400		0		0
Impaired loans, average recorded investment	15,053		867
Impaired loans, interest income recognized	0		0
Commercial | Other commercial and industrial [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	27,421		28,876		27,957
Impaired loans, recorded investment, total	18,501		20,781		19,098
Impaired loans, recorded investment with no related allowance	18,501		20,764		19,080
Impaired loans, recorded investment with related allowance	0		17		18
Impaired loans, related allowance	0		17		17
Impaired loans, average recorded investment	18,799		20,950
Impaired loans, interest income recognized	0		0
Commercial real estate
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	4,366		7,453		4,751
Impaired loans, recorded investment, total	2,470		4,475		2,855
Impaired loans, recorded investment with no related allowance	2,470		4,475		2,855
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	2,663		4,998
Impaired loans, interest income recognized	0		0
Commercial real estate | Residential construction and land development [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	3,059		4,126		3,285
Impaired loans, recorded investment, total	1,613		2,616		1,832
Impaired loans, recorded investment with no related allowance	1,613		2,616		1,832
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	1,723		3,024
Impaired loans, interest income recognized	0		0
Commercial real estate | Retail [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	498		523		509
Impaired loans, recorded investment, total	264		314		276
Impaired loans, recorded investment with no related allowance	264		314		276
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	270		320
Impaired loans, interest income recognized	0		0
Commercial real estate | Office [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	287		515		287
Impaired loans, recorded investment, total	275		413		275
Impaired loans, recorded investment with no related allowance	275		413		275
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	275		420
Impaired loans, interest income recognized	0		0
Commercial real estate | Multifamily [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	0		1,000		0
Impaired loans, recorded investment, total	0		24		0
Impaired loans, recorded investment with no related allowance	0		24		0
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	0		31
Impaired loans, interest income recognized	0		0
Commercial real estate | Industrial [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	0		76		0
Impaired loans, recorded investment, total	0		76		0
Impaired loans, recorded investment with no related allowance	0		76		0
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	0		76
Impaired loans, interest income recognized	0		0
Commercial real estate | Other commercial real estate [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	522		1,213		670
Impaired loans, recorded investment, total	318		1,032		472
Impaired loans, recorded investment with no related allowance	318		1,032		472
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	395		1,127
Impaired loans, interest income recognized	0		0
Residential mortgage
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	227,060		252,600		248,797
Impaired loans, recorded investment, total	214,791		240,154		235,774
Impaired loans, recorded investment with no related allowance	214,791		240,109		235,774
Impaired loans, recorded investment with related allowance	0		45		0
Impaired loans, related allowance	0		45		0
Impaired loans, average recorded investment	236,969		242,569
Impaired loans, interest income recognized	2,154		2,195
Residential mortgage | Permanent mortgage [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	29,686		29,355		30,435
Impaired loans, recorded investment, total	24,578		24,188		25,193
Impaired loans, recorded investment with no related allowance	24,578		24,143		25,193
Impaired loans, recorded investment with related allowance	0		45		0
Impaired loans, related allowance	0		45		0
Impaired loans, average recorded investment	24,885		23,521
Impaired loans, interest income recognized	306		291
Residential mortgage | Permanent mortgages guaranteed by US government agencies [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	183,476	[1]	210,237	[2]	203,814	[3]
Impaired loans, recorded investment, total	177,880	[1]	204,181	[2]	197,506	[3]
Impaired loans, recorded investment with no related allowance	177,880	[1]	204,181	[2]	197,506	[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	199,380	[1]	207,396	[2]
Impaired loans, interest income recognized	1,848	[1]	1,904	[2]
Residential mortgage | Home equity [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	13,898		13,008		14,548
Impaired loans, recorded investment, total	12,333		11,785		13,075
Impaired loans, recorded investment with no related allowance	12,333		11,785		13,075
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		0
Impaired loans, average recorded investment	12,704		11,652
Impaired loans, interest income recognized	0		0
Personal
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	381		265		307
Impaired loans, recorded investment, total	340		235		269
Impaired loans, recorded investment with no related allowance	340		235		269
Impaired loans, recorded investment with related allowance	0		0		0
Impaired loans, related allowance	0		0		$ 0
Impaired loans, average recorded investment	304		262
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, 2018, $8.9 million of these loans were nonaccruing and $169 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, 2017, $10 million of these loans were nonaccruing and $194 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 million of these loans were nonaccruing and $188 million were accruing based on the guarantee by U.S. government agencies.