Loans and Allowance for Credit Losses, Impaired Loans (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2018		Sep. 30, 2017		Sep. 30, 2018		Sep. 30, 2017		Dec. 31, 2017
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	$ 392,990		$ 460,466		$ 392,990		$ 460,466		$ 438,459
Impaired loans, recorded investment, total	325,920		404,115		325,920		404,115		376,201
Impaired loans, recorded investment with no related allowance	287,442		314,610		287,442		314,610		324,867
Impaired loans, recorded investment with related allowance	38,478		89,505		38,478		89,505		51,334
Impaired loans, related allowance	13,797		13,284		13,797		13,284		8,831
Impaired loans, average recorded investment	326,300		415,027		340,591		417,476
Impaired loans, interest income recognized	1,875		2,199		5,926		6,721
Commercial [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	156,620		218,386		156,620		218,386		184,604
Impaired loans, recorded investment, total	109,490		176,900		109,490		176,900		137,303
Impaired loans, recorded investment with no related allowance	71,012		87,395		71,012		87,395		85,969
Impaired loans, recorded investment with related allowance	38,478		89,505		38,478		89,505		51,334
Impaired loans, related allowance	13,797		13,284		13,797		13,284		8,831
Impaired loans, average recorded investment	115,234		187,029		123,396		177,927
Impaired loans, interest income recognized	0		0		0		0
Commercial [Member] | Energy [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	73,600		133,643		73,600		133,643		111,011
Impaired loans, recorded investment, total	54,033		110,683		54,033		110,683		92,284
Impaired loans, recorded investment with no related allowance	28,180		45,169		28,180		45,169		40,968
Impaired loans, recorded investment with related allowance	25,853		65,514		25,853		65,514		51,316
Impaired loans, related allowance	5,305		4,944		5,305		4,944		8,814
Impaired loans, average recorded investment	59,815		117,338		73,159		121,591
Impaired loans, interest income recognized	0		0		0		0
Commercial [Member] | Services [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	6,959		3,838		6,959		3,838		5,324
Impaired loans, recorded investment, total	4,097		1,174		4,097		1,174		2,620
Impaired loans, recorded investment with no related allowance	4,021		1,174		4,021		1,174		2,620
Impaired loans, recorded investment with related allowance	76		0		76		0		0
Impaired loans, related allowance	76		0		76		0		0
Impaired loans, average recorded investment	4,237		4,464		3,358		4,674
Impaired loans, interest income recognized	0		0		0		0
Commercial [Member] | Wholesale/retail [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	14,281		8,418		14,281		8,418		9,099
Impaired loans, recorded investment, total	9,249		1,893		9,249		1,893		2,574
Impaired loans, recorded investment with no related allowance	2,227		1,893		2,227		1,893		2,574
Impaired loans, recorded investment with related allowance	7,022		0		7,022		0		0
Impaired loans, related allowance	4,102		0		4,102		0		0
Impaired loans, average recorded investment	11,672		6,256		5,911		6,650
Impaired loans, interest income recognized	0		0		0		0
Commercial [Member] | Manufacturing [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	9,212	[1]	9,674		9,212	[1]	9,674		6,073
Impaired loans, recorded investment, total	9,202	[1]	9,059		9,202	[1]	9,059		5,962
Impaired loans, recorded investment with no related allowance	6,217	[1]	9,059		6,217	[1]	9,059		5,962
Impaired loans, recorded investment with related allowance	2,985	[1]	0		2,985	[1]	0		0
Impaired loans, related allowance	2,985	[1]	0		2,985	[1]	0		0
Impaired loans, average recorded investment	6,096	[1]	9,357		7,582	[1]	6,995
Impaired loans, interest income recognized	0	[1]	0		0	[1]	0
Impaired loans, unpaid principal balance to affiliates	6,200				6,200
Commercial [Member] | Healthcare [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	25,923		24,591		25,923		24,591		25,140
Impaired loans, recorded investment, total	15,704		24,446		15,704		24,446		14,765
Impaired loans, recorded investment with no related allowance	13,162		474		13,162		474		14,765
Impaired loans, recorded investment with related allowance	2,542		23,972		2,542		23,972		0
Impaired loans, related allowance	1,329		8,323		1,329		8,323		0
Impaired loans, average recorded investment	15,915		24,476		15,235		12,635
Impaired loans, interest income recognized	0		0		0		0
Commercial [Member] | Other commercial and industrial [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	26,645		38,222		26,645		38,222		27,957
Impaired loans, recorded investment, total	17,205		29,645		17,205		29,645		19,098
Impaired loans, recorded investment with no related allowance	17,205		29,626		17,205		29,626		19,080
Impaired loans, recorded investment with related allowance	0		19		0		19		18
Impaired loans, related allowance	0		17		0		17		17
Impaired loans, average recorded investment	17,499		25,138		18,151		25,382
Impaired loans, interest income recognized	0		0		0		0
Commercial real estate [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	9,611		5,003		9,611		5,003		4,751
Impaired loans, recorded investment, total	1,316		2,975		1,316		2,975		2,855
Impaired loans, recorded investment with no related allowance	1,316		2,975		1,316		2,975		2,855
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	1,656		3,375		2,085		4,250
Impaired loans, interest income recognized	0		0		0		0
Commercial real estate [Member] | Residential construction and land development [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	1,306		3,532		1,306		3,532		3,285
Impaired loans, recorded investment, total	350		1,924		350		1,924		1,832
Impaired loans, recorded investment with no related allowance	350		1,924		350		1,924		1,832
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	350		1,988		1,091		2,679
Impaired loans, interest income recognized	0		0		0		0
Commercial real estate [Member] | Retail [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	7,951		513		7,951		513		509
Impaired loans, recorded investment, total	777		289		777		289		276
Impaired loans, recorded investment with no related allowance	777		289		777		289		276
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	923		295		527		308
Impaired loans, interest income recognized	0		0		0		0
Commercial real estate [Member] | Office [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	0		287		0		287		287
Impaired loans, recorded investment, total	0		275		0		275		275
Impaired loans, recorded investment with no related allowance	0		275		0		275		275
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	137		335		137		351
Impaired loans, interest income recognized	0		0		0		0
Commercial real estate [Member] | Multifamily [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	0		0		0		0		0
Impaired loans, recorded investment, total	0		0		0		0		0
Impaired loans, recorded investment with no related allowance	0		0		0		0		0
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	0		5		0		19
Impaired loans, interest income recognized	0		0		0		0
Commercial real estate [Member] | Industrial [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	0		0		0		0		0
Impaired loans, recorded investment, total	0		0		0		0		0
Impaired loans, recorded investment with no related allowance	0		0		0		0		0
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	0		0		0		38
Impaired loans, interest income recognized	0		0		0		0
Commercial real estate [Member] | Other commercial real estate [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	354		671		354		671		670
Impaired loans, recorded investment, total	189		487		189		487		472
Impaired loans, recorded investment with no related allowance	189		487		189		487		472
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	246		752		330		855
Impaired loans, interest income recognized	0		0		0		0
Residential mortgage [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	226,439		236,787		226,439		236,787		248,797
Impaired loans, recorded investment, total	214,845		223,985		214,845		223,985		235,774
Impaired loans, recorded investment with no related allowance	214,845		223,985		214,845		223,985		235,774
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	209,105		224,360		214,841		235,026
Impaired loans, interest income recognized	1,875		2,199		5,926		6,721
Residential mortgage [Member] | Permanent mortgage [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	27,603		29,861		27,603		29,861		30,435
Impaired loans, recorded investment, total	22,855		24,623		22,855		24,623		25,193
Impaired loans, recorded investment with no related allowance	22,855		24,623		22,855		24,623		25,193
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	22,980		24,019		24,024		23,739
Impaired loans, interest income recognized	318		315		947		912
Residential mortgage [Member] | Permanent mortgages guaranteed by US government agencies [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	185,788	[2]	193,594	[3]	185,788	[2]	193,594	[3]	203,814	[4]
Impaired loans, recorded investment, total	180,718	[2]	187,370	[3]	180,718	[2]	187,370	[3]	197,506	[4]
Impaired loans, recorded investment with no related allowance	180,718	[2]	187,370	[3]	180,718	[2]	187,370	[3]	197,506	[4]
Impaired loans, recorded investment with related allowance	0	[2]	0	[3]	0	[2]	0	[3]	0	[4]
Impaired loans, related allowance	0	[2]	0	[3]	0	[2]	0	[3]	0	[4]
Impaired loans, average recorded investment	174,653	[2]	188,461	[3]	178,643	[2]	199,532	[3]
Impaired loans, interest income recognized	1,557	[2]	1,884	[3]	4,979	[2]	5,809	[3]
Residential mortgage [Member] | Home equity [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	13,048		13,332		13,048		13,332		14,548
Impaired loans, recorded investment, total	11,272		11,992		11,272		11,992		13,075
Impaired loans, recorded investment with no related allowance	11,272		11,992		11,272		11,992		13,075
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	11,472		11,880		12,174		11,755
Impaired loans, interest income recognized	0		0		0		0
Personal [Member]
Impaired loans [Abstract]
Impaired loans, unpaid principal balance	320		290		320		290		307
Impaired loans, recorded investment, total	269		255		269		255		269
Impaired loans, recorded investment with no related allowance	269		255		269		255		269
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	305		263		269		273
Impaired loans, interest income recognized	$ 0		$ 0		$ 0		$ 0

[1] Impaired manufacturing sector loans included $6.2 million of loans from an affiliated entity, with no allowance as the fair value of the collateral exceeded the outstanding principal balance at September 30, 2018.

[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 September 30, 2018, $7.8 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 September 30, 2017, $8.9 million of these loans were nonaccruing and $178 million were accruing based on the guarantee by U.S. government agencies.

[4] 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, $9.2 million of these loans were nonaccruing and $188 million were accruing based on the guarantee by U.S. government agencies.