Securities - Amortized Cost, Estimated Fair Value and Credit Rating of Corporate Debt Securities (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 704,767	$ 607,833
Estimated Fair Value	695,564	$ 598,119
BankAmerica Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	15,000
Estimated Fair Value	$ 13,513
Credit Rating Moody’s/ S&P	Ba1/BB+
Chase Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 10,000
Estimated Fair Value	$ 9,055
Credit Rating Moody’s/ S&P	Baa2/BBB-
Wells Fargo Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Estimated Fair Value	$ 4,513
Credit Rating Moody’s/ S&P	A1/BBB+
Huntington Capital
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
Keycorp Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Estimated Fair Value	$ 4,425
Credit Rating Moody’s/ S&P	Baa2/BB+
PNC Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Estimated Fair Value	$ 4,525
Credit Rating Moody’s/ S&P	Baa1/BBB-
State Street Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Estimated Fair Value	$ 4,500
Credit Rating Moody’s/ S&P	A3/BBB
SunTrust Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Estimated Fair Value	$ 4,362
Credit Rating Moody’s/ S&P	Not Rated/BB+
Corporate Debt Securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 55,000
Estimated Fair Value	$ 49,118