Securities - Amortized Cost, Estimated Fair Value and Credit Rating of Corporate Debt Securities (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 823,717	$ 851,903
Estimated Fair Value	826,964	$ 832,815
Chase Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	10,000
Estimated Fair Value	$ 9,200
Credit Rating Moody’s/ S&P	Baa1/BBB-
Wells Fargo Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Estimated Fair Value	$ 4,625
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	$ 4,450
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	$ 4,525
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	$ 4,600
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	$ 4,613
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	$ 4,625
Credit Rating Moody’s/ S&P	Not Rated/BB+
Celgene
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,511
Estimated Fair Value	$ 1,508
Credit Rating Moody’s/ S&P	Baa2/BBB+
Southern Company
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,507
Estimated Fair Value	$ 1,505
Credit Rating Moody’s/ S&P	Baa2/BBB+
BB&T
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,506
Estimated Fair Value	$ 1,505
Credit Rating Moody’s/ S&P	A2/A-
AT&T Inc.
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,505
Estimated Fair Value	$ 1,504
Credit Rating Moody’s/ S&P	Baa2/BBB
Corporate debt securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 46,029
Estimated Fair Value	$ 42,660