Loans, Changes in the Allowance for Loan Losses by Portfolio Segment (Details) - USD ($) $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2019		Dec. 31, 2018
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of period		$ 238,023	$ 265,880
Charge offs		(31,247)	(50,536)
Recoveries		10,384	20,179
Net Charge offs		(20,864)	(30,358)
Provision for loan losses		16,500	2,500
Balance at end of period		233,659	238,023
Allowance for loan losses:
Individually evaluated				$ 32,188		$ 15,304
Collectively evaluated				201,470		222,719
Total allowance for loan losses		238,023	238,023	233,659		238,023
Loans:
Individually evaluated				172,489		133,690
Collectively evaluated				23,075,334		22,801,887
Acquired and accounted for under ASC 310-30				2,145	[1]	4,853
Total loans	[2]			23,249,967		22,940,429
Commercial and industrial
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of period		108,835	123,068
Charge offs		(26,255)	(30,837)
Recoveries		6,650	13,714
Net Charge offs		(19,605)	(17,123)
Provision for loan losses		23,970	2,890
Balance at end of period		113,199	108,835
Allowance for loan losses:
Individually evaluated				22,890		5,721
Collectively evaluated				90,309		103,114
Total allowance for loan losses		113,199	123,068	113,199		108,835
Loans:
Individually evaluated				100,066		63,153
Collectively evaluated				7,478,790		7,331,898
Acquired and accounted for under ASC 310-30				528	[1]	2,994
Total loans				7,579,384		7,398,044
Commercial real estate — owner occupied
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of period		9,255	10,352
Charge offs		(222)	(1,363)
Recoveries		1,312	639
Net Charge offs		1,089	(724)
Provision for loan losses		(1,110)	(373)
Balance at end of period		9,235	9,255
Allowance for loan losses:
Individually evaluated				27		24
Collectively evaluated				9,207		9,231
Total allowance for loan losses		9,235	10,352	9,235		9,255
Loans:
Individually evaluated				2,473		5,852
Collectively evaluated				939,647		913,708
Acquired and accounted for under ASC 310-30				691	[1]	883
Total loans				942,811		920,443
Commercial real estate — investor
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of period		40,844	41,059
Charge offs		0	(7,914)
Recoveries		34	668
Net Charge offs		34	(7,246)
Provision for loan losses		10	7,031
Balance at end of period		40,888	40,844
Allowance for loan losses:
Individually evaluated				103		28
Collectively evaluated				40,785		40,816
Total allowance for loan losses		40,888	41,059	40,888		40,844
Loans:
Individually evaluated				1,411		2,384
Collectively evaluated				3,777,511		3,748,883
Acquired and accounted for under ASC 310-30				279	[1]	287
Total loans				3,779,201		3,751,554
Real estate construction
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of period		28,240	34,370
Charge offs		(60)	(298)
Recoveries		211	446
Net Charge offs		151	149
Provision for loan losses		(1,267)	(6,279)
Balance at end of period		27,124	28,240
Allowance for loan losses:
Individually evaluated				56		75
Collectively evaluated				27,068		28,165
Total allowance for loan losses		28,240	28,240	27,124		28,240
Loans:
Individually evaluated				417		510
Collectively evaluated				1,394,384		1,334,500
Acquired and accounted for under ASC 310-30				14	[1]	21
Total loans				1,394,815		1,335,031
Residential mortgage
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of period		25,595	29,607
Charge offs		(1,260)	(1,627)
Recoveries		438	1,271
Net Charge offs		(822)	(355)
Provision for loan losses		(3,421)	(3,657)
Balance at end of period		21,352	25,595
Allowance for loan losses:
Individually evaluated				5,693		6,023
Collectively evaluated				15,659		19,572
Total allowance for loan losses		25,595	25,595	21,352		25,595
Loans:
Individually evaluated				56,272		50,486
Collectively evaluated				8,220,602		8,226,642
Acquired and accounted for under ASC 310-30				605	[1]	584
Total loans				8,277,479		8,277,712
Home equity
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of period		19,266	22,126
Charge offs		(725)	(3,236)
Recoveries		1,273	2,628
Net Charge offs		548	(608)
Provision for loan losses		(4,241)	(2,252)
Balance at end of period		15,573	19,266
Allowance for loan losses:
Individually evaluated				3,233		3,312
Collectively evaluated				12,340		15,954
Total allowance for loan losses		19,266	19,266	15,573		19,266
Loans:
Individually evaluated				10,626		10,124
Collectively evaluated				905,560		884,266
Acquired and accounted for under ASC 310-30				27	[1]	83
Total loans				916,213		894,473
Other consumer
Changes in the allowance for loan losses by portfolio segment
Balance at beginning of period		5,988	5,298
Charge offs		(2,726)	(5,261)
Recoveries		467	812
Net Charge offs		(2,259)	(4,448)
Provision for loan losses		2,558	5,138
Balance at end of period		6,287	5,988
Allowance for loan losses:
Individually evaluated				185		121
Collectively evaluated				6,102		5,867
Total allowance for loan losses		$ 6,287	$ 5,298	6,287		5,988
Loans:
Individually evaluated				1,225		1,181
Collectively evaluated				358,841		361,990
Acquired and accounted for under ASC 310-30				0	[1]	0
Total loans				$ 360,065		$ 363,171

[1] (a) Loans acquired in business combinations and accounted for under ASC Subtopic 310-30 "Receivables — Loans and Debt Securities Acquired with Deteriorated Credit Quality."

[2] (a) Includes $2 million and $5 million of purchased credit-impaired loans at June 30, 2019 and December 31, 2018, respectively.