Intangible Assets - Summary of Intangible Assets (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 2,450
Goodwill not subject to amortization	1,794,779	$ 1,478,014
Core Deposit Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	101,396	98,359
Accumulated Amortization	(72,738)	(69,508)
George Mason Trade Name Intangible [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,080	1,080
Crescent Mortgage Trade Name [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,370
Community Banking [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	0
Goodwill not subject to amortization	1,789,464	1,472,699
Community Banking [Member] | Core Deposit Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	101,396	98,359
Accumulated Amortization	(72,738)	(69,508)
Community Banking [Member] | George Mason Trade Name Intangible [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	0	0
Community Banking [Member] | Crescent Mortgage Trade Name [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	0
Mortgage Banking [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	2,450
Goodwill not subject to amortization	5,315	5,315
Mortgage Banking [Member] | Core Deposit Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	0	0
Accumulated Amortization	0	0
Mortgage Banking [Member] | George Mason Trade Name Intangible [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,080	$ 1,080
Mortgage Banking [Member] | Crescent Mortgage Trade Name [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 1,370