Securities - Amortized Cost, Estimated Fair Value and Credit Rating of Corporate Debt Securities (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Schedule of Held-to-maturity Securities [Line Items]
Estimated Fair Value	$ 474,215	$ 495,082
BankAmerica Capital [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	15,000
Estimated Fair Value	12,000
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	4,200
Credit Rating Moody's/S&P	A3/BBB+
Huntington Capital [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,000
Estimated Fair Value	4,000
Credit Rating Moody's/S&P	Baa3/BB
Keycorp Capital [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,000
Estimated Fair Value	4,100
Credit Rating Moody's/S&P	Baa3/BB+
PNC Capital [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,000
Estimated Fair Value	4,200
Credit Rating Moody's/S&P	Baa2/BBB-
State Street Capital [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,000
Estimated Fair Value	4,250
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	4,100
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	$ 45,250