Mergers and Acquisitions - Schedule of Fair Value of Acquired Identifiable Assets and Liabilities Assumed (Detail) - USD ($)	Apr. 21, 2017	Jun. 03, 2016	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Resulting goodwill			$ 1,478,380,000	$ 863,767,000
Cardinal Financial Corporation [Member]
Business Acquisition [Line Items]
Value of common shares issued	$ 972,499,000
Fair value of stock options assumed	2,741,000
Cash for fractional shares	14,000
Total purchase price	975,254,000
Cash and cash equivalents	44,545,000
Investment securities	395,829,000
Loans held for sale	271,301,000
Loans	3,168,599,000
Premises and equipment	24,208,000
Other assets	135,383,000
Total identifiable assets	4,069,669,000
Deposits	3,349,812,000
Short-term borrowings	96,215,000
Long-term borrowings	220,119,000
Unfavorable lease liability	2,281,000
Other liabilities	39,274,000
Total identifiable liabilities	3,707,701,000
Fair value of net assets acquired including identifiable intangible assets	361,968,000
Resulting goodwill	613,286,000
Cardinal Financial Corporation [Member] | Core Deposit Intangible Assets [Member]
Business Acquisition [Line Items]
Cash for fractional shares	14,000
Core deposit intangibles	28,724,000
Cardinal Financial Corporation [Member] | George Mason Trade Name Intangible [Member]
Business Acquisition [Line Items]
Core deposit intangibles	$ 1,080,000
Bank of Georgetown [Member]
Business Acquisition [Line Items]
Value of common shares issued		$ 253,799,000
Fair value of stock options assumed		10,696,000
Cash for fractional shares		10,000
Total purchase price		264,505,000
Cash and cash equivalents		29,340,000
Investment securities		219,783,000
Loans		966,870,000
Premises and equipment		5,574,000
Other assets		31,605,000
Total identifiable assets		1,262,230,000
Deposits		971,685,000
Short-term borrowings		101,021,000
Long-term borrowings		67,659,000
Other liabilities		11,532,000
Total identifiable liabilities		1,151,897,000
Fair value of net assets acquired including identifiable intangible assets		110,333,000
Resulting goodwill		154,172,000
Bank of Georgetown [Member] | Core Deposit Intangible Assets [Member]
Business Acquisition [Line Items]
Cash for fractional shares		10,000
Core deposit intangibles		$ 9,058,000