Securities - Amortized Cost, Estimated Fair Value and Credit Rating of Corporate Debt Securities (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 425,904	$ 481,340
Estimated Fair Value	420,409	$ 474,215
BankAmerica Capital [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	15,000
Estimated Fair Value	$ 13,006
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	$ 8,625
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	$ 4,344
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	$ 4,025
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	$ 4,225
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	$ 4,450
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	$ 4,338
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	$ 4,237
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	$ 47,250