Securities - Amortized Cost, Estimated Fair Value and Credit Rating of Corporate Debt Securities (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost		$ 888,848	$ 772,060
Held-to-maturity, Estimated Fair Value	$ 864,173	$ 761,660
Chase Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	10,000
Held-to-maturity, Estimated Fair Value	$ 9,350
Credit Rating Moody’s/ S&P	Baa2/BBB-
Wells Fargo Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Held-to-maturity, Estimated Fair Value	$ 4,650
Credit Rating Moody’s/ S&P	A1/BBB
Huntington Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Held-to-maturity, Estimated Fair Value	$ 4,525
Credit Rating Moody’s/ S&P	Baa2/BB+
Keycorp Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Held-to-maturity, Estimated Fair Value	$ 4,600
Credit Rating Moody’s/ S&P	Baa2/BB+
PNC Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Held-to-maturity, Estimated Fair Value	$ 4,678
Credit Rating Moody’s/ S&P	Baa1/BBB-
State Street Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Held-to-maturity, Estimated Fair Value	$ 4,700
Credit Rating Moody’s/ S&P	A3/BBB
SunTrust Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Held-to-maturity, Estimated Fair Value	$ 4,625
Credit Rating Moody’s/ S&P	Not Rated/BB+
Southern Company
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,516
Held-to-maturity, Estimated Fair Value	$ 1,485
Credit Rating Moody’s/ S&P	Baa2/BBB+
AT&T Inc.
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,510
Held-to-maturity, Estimated Fair Value	$ 1,480
Credit Rating Moody’s/ S&P	Baa2/BBB
Celgene
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,524
Held-to-maturity, Estimated Fair Value	$ 1,490
Credit Rating Moody’s/ S&P	Baa2/BBB+
Corporate Debt Securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 44,550
Held-to-maturity, Estimated Fair Value	$ 41,583