Securities - Amortized Cost, Estimated Fair Value and Credit Rating of Corporate Debt Securities (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 523,590	$ 405,361
Estimated Fair Value	520,971	$ 397,763
BankAmerica Capital [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	15,000
Estimated Fair Value	$ 12,350
Credit Rating Moody's/S&P	Ba1/BB+
Chase Capital [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 10,000
Estimated Fair Value	$ 8,400
Credit Rating Moody's/S&P	Baa2/BBB-
Wells Fargo Capital [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Estimated Fair Value	$ 4,213
Credit Rating Moody's/S&P	A1/BBB+
Huntington Capital [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Estimated Fair Value	$ 3,800
Credit Rating Moody's/S&P	Baa2/BB
Keycorp Capital [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Estimated Fair Value	$ 4,019
Credit Rating Moody's/S&P	Baa2/BB+
PNC Capital [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Estimated Fair Value	$ 4,400
Credit Rating Moody's/S&P	Baa1/BBB-
State Street Capital [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Estimated Fair Value	$ 4,262
Credit Rating Moody's/S&P	A3/BBB
SunTrust Capital [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Estimated Fair Value	$ 3,962
Credit Rating Moody's/S&P	Baa3/BB+
Corporate Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 55,000
Estimated Fair Value	$ 45,406