Mergers and Acquisitions - Additional Information (Detail) - USD ($)			6 Months Ended
	Jun. 30, 2018	Apr. 21, 2017	Jun. 30, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Total assets at consummation	$ 19,207,603,000		$ 19,207,603,000	$ 19,058,959,000
Loans at consummation	13,529,008,000		13,529,008,000	13,027,337,000
Deposits at consummation	13,830,766,000		13,830,766,000	13,830,591,000
Resulting goodwill	1,478,014,000		1,478,014,000	$ 1,478,380,000
Liabilities to provide severance benefits	$ 0		$ 0
Contractually required principal and interest at acquisition		$ 132,837,000
Expected cash flows at acquisition		108,275,000
Estimated fair value of acquired impaired loans at acquisition		$ 86,696,000
Cardinal Financial Corporation [Member]
Business Acquisition [Line Items]
Acquisition completion date			Apr. 21, 2017
Outstanding common stock acquired		100.00%
Total assets at consummation		$ 4,136,008,000
Loans at consummation		3,313,033,000
Deposits at consummation		3,344,740,000
Common stock paid in cash		975,254,000
Common stock value		972,499,000
Stock options exchanged value		$ 2,741,000
Common stock, shares issued		23,690,589
Resulting goodwill		$ 612,920,000
Fractional shares cash paid per share		14,000
Goodwill from acquisition expected to be tax deductible		0
Fair value discounts on loans acquired		144,434,000
Fair value discounts on leases		2,281,000
Fair value adjustment on trust preferred issuances		8,738,000
Premiums on land acquired		4,408,000
Premiums on interest-bearing deposits acquired		5,072,000
Premiums on long-term FHLB advances acquired		10,740,000
Estimated remaining life of discount on leases acquired	5 years 2 months 30 days
Estimated remaining life of discount on trust preferred issuances acquired	16 years 2 months 19 days
Estimated remaining life of premium on interest-bearing deposits acquired	4 years 2 months 30 days
Estimated remaining life of the FHLB advances acquired	4 years 22 days
Assumed liabilities to provide severance benefits		1,825,000
Contractually required principal and interest at acquisition		4,211,734,000
Expected cash flows at acquisition		4,155,558,000
Estimated fair value of acquired impaired loans at acquisition		3,168,599,000
Revenue since acquisition included in consolidated results			$ 431,463,000
Revenue since acquisition included in consolidated results			$ 212,834,000
Cardinal Financial Corporation [Member] | Common Stock [Member]
Business Acquisition [Line Items]
Common stock value		$ 972,499,000
Common stock, shares issued		23,690,589
Closing market price per common share		$ 41.05
Cardinal Financial Corporation [Member] | Core Deposit Intangible Assets [Member]
Business Acquisition [Line Items]
Purchase price allocation of Identifiable Intangible assets		$ 28,724,000
Estimated period of amortization of core deposit intangibles		10 years
Fractional shares cash paid per share		$ 14,000
Cardinal Financial Corporation [Member] | George Mason Trade Name Intangible [Member]
Business Acquisition [Line Items]
Purchase price allocation of Identifiable Intangible assets		$ 1,080,000