Intangible Assets- Summary of Intangible Assets (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Goodwill not subject to amortization	$ 1,478,014	$ 1,478,380
Core Deposit Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	98,359	98,359
Accumulated Amortization	(58,473)	(54,453)
George Mason Trade Name Intangible [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,080	1,080
Community Banking [Member]
Finite-Lived Intangible Assets [Line Items]
Goodwill not subject to amortization	1,472,699	1,473,265
Community Banking [Member] | Core Deposit Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	98,359	98,359
Accumulated Amortization	(58,473)	(54,453)
Community Banking [Member] | George Mason Trade Name Intangible [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	0	0
Mortgage Banking [Member]
Finite-Lived Intangible Assets [Line Items]
Goodwill not subject to amortization	5,315	5,115
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