Allowance for Credit Losses - Allocation of Allowance for Loan Losses and Related Loans by Segment (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Allowance for loan losses:
Individually evaluated for impairment	$ 37,599		$ 33,025
Collectively evaluated for impairment	124,254		118,834
Total	161,853	$ 155,105	151,859	$ 144,963	$ 138,762	$ 120,856
Individually evaluated for impairment	173,054		156,625
Collectively evaluated for impairment	22,856,937		20,688,758
Total	26,567,159		25,035,469
Loans acquired with discounts related to credit quality
Allowance for loan losses:
Total	3,537,168		4,190,086
Commercial and Industrial
Allowance for loan losses:
Individually evaluated for impairment	35,730		29,684
Collectively evaluated for impairment	65,272		61,272
Total	101,002	94,384	90,956	83,997	74,257	57,232
Individually evaluated for impairment	110,539		97,852
Collectively evaluated for impairment	3,927,525		3,492,523
Total	4,695,608		4,331,032
Commercial and Industrial | Loans acquired with discounts related to credit quality
Allowance for loan losses:
Total	657,544		740,657
Commercial Real Estate
Allowance for loan losses:
Individually evaluated for impairment	1,287		2,628
Collectively evaluated for impairment	47,484		47,022
Total	48,771	48,978	49,650	50,130	53,812	54,954
Individually evaluated for impairment	50,124		43,598
Collectively evaluated for impairment	12,456,271		10,991,059
Total	14,903,044		13,895,407
Commercial Real Estate | Loans acquired with discounts related to credit quality
Allowance for loan losses:
Total	2,396,649		2,860,750
Residential Mortgage
Allowance for loan losses:
Individually evaluated for impairment	525		600
Collectively evaluated for impairment	4,777		4,441
Total	5,302	5,219	5,041	4,919	4,624	3,605
Individually evaluated for impairment	11,656		13,904
Collectively evaluated for impairment	3,758,844		3,669,080
Total	4,133,331		4,111,400
Residential Mortgage | Loans acquired with discounts related to credit quality
Allowance for loan losses:
Total	362,831		428,416
Consumer
Allowance for loan losses:
Individually evaluated for impairment	57		113
Collectively evaluated for impairment	6,721		6,099
Total	6,778	$ 6,524	6,212	$ 5,917	$ 6,069	$ 5,065
Individually evaluated for impairment	735		1,271
Collectively evaluated for impairment	2,714,297		2,536,096
Total	2,835,176		2,697,630
Consumer | Loans acquired with discounts related to credit quality
Allowance for loan losses:
Total	$ 120,144		$ 160,263