Loans and Allowance for Credit Losses for Loans - Summary of Allocation of Allowance for Loan Losses and Related Loans by Loan Portfolio Segment Disaggregated Based on Allowance Measurement Methodology (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually evaluated for impairment	$ 70,564		$ 38,576
Collectively evaluated for impairment	254,468		123,183
Total	325,032	$ 309,614	161,759	$ 161,853	$ 155,105	$ 151,859
Individually evaluated for impairment	246,411		163,644
Collectively evaluated for impairment	32,169,175		22,905,965
Total	32,415,586		29,699,208
Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Loss [Line Items]
Total			6,629,599
Commercial and Industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually evaluated for impairment	66,444		36,662
Collectively evaluated for impairment	63,965		67,397
Total	130,409	132,039	104,059	101,002	94,384	90,956
Individually evaluated for impairment	136,696		100,860
Collectively evaluated for impairment	6,766,649		4,043,123
Total	6,903,345		4,825,997
Commercial and Industrial | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Loss [Line Items]
Total			682,014
Commercial Real Estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually evaluated for impairment	2,058		1,338
Collectively evaluated for impairment	142,592		44,335
Total	144,650	131,702	45,673	48,771	48,978	49,650
Individually evaluated for impairment	70,145		51,242
Collectively evaluated for impairment	18,466,217		12,347,368
Total	18,536,362		17,643,759
Commercial Real Estate | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Loss [Line Items]
Total			5,245,149
Residential Mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually evaluated for impairment	830		518
Collectively evaluated for impairment	27,784		4,542
Total	28,614	29,630	5,060	5,302	5,219	5,041
Individually evaluated for impairment	35,551		10,689
Collectively evaluated for impairment	4,249,044		3,786,253
Total	4,284,595		4,377,111
Residential Mortgage | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Loss [Line Items]
Total			580,169
Consumer
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually evaluated for impairment	1,232		58
Collectively evaluated for impairment	20,127		6,909
Total	21,359	$ 16,243	6,967	$ 6,778	$ 6,524	$ 6,212
Individually evaluated for impairment	4,019		853
Collectively evaluated for impairment	2,687,265		2,729,221
Total	$ 2,691,284		2,852,341
Consumer | Loans acquired with discounts related to credit quality
Financing Receivable, Allowance for Credit Loss [Line Items]
Total			$ 122,267