Securities - Amortized Cost, Estimated Fair Value and Credit Rating of Corporate Debt Securities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 851,903	$ 772,060
Estimated Fair Value	832,815	$ 761,660
Chase Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	10,000
Estimated Fair Value	$ 8,800
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	$ 4,300
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	$ 4,350
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	$ 4,400
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	$ 4,570
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	$ 4,475
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	$ 4,300
Credit Rating Moody's/S&P	Not Rated/BB+
Southern Company
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,514
Estimated Fair Value	$ 1,483
Credit Rating Moody's/S&P	Baa2/BBB+
AT&T Inc.
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,509
Estimated Fair Value	$ 1,480
Credit Rating Moody's/S&P	Baa2/BBB
BB&T
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,512
Estimated Fair Value	$ 1,487
Credit Rating Moody's/S&P	A2/A-
Celgene
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,521
Estimated Fair Value	$ 1,490
Credit Rating Moody's/S&P	Baa2/BBB+
Haliburton
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 461
Estimated Fair Value	$ 436
Credit Rating Moody's/S&P	Baa1/A-
Ryder
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,504
Estimated Fair Value	$ 1,488
Credit Rating Moody's/S&P	Baa1/BBB+
Corporate Debt Securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 48,021
Estimated Fair Value	$ 43,059