Securities - Amortized Cost, Estimated Fair Value and Credit Rating of Corporate Debt Securities (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost		$ 990,582	$ 772,060
Held-to-maturity, Estimated Fair Value		$ 761,660
BankAmerica Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	15,000
Held-to-maturity, Estimated Fair Value	$ 14,044
Credit Rating Moody’s/ S&P	Baa3/BBB-
Chase Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 10,000
Held-to-maturity, Estimated Fair Value	$ 9,250
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	$ 4,725
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	$ 4,663
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	$ 4,700
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	$ 4,734
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	$ 4,642
Credit Rating Moody’s/ S&P	Not Rated/BB+
Southern Company
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,521
Held-to-maturity, Estimated Fair Value	$ 1,487
Credit Rating Moody’s/ S&P	Baa2/BBB+
AT&T Inc.
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,513
Held-to-maturity, Estimated Fair Value	$ 1,483
Credit Rating Moody’s/ S&P	Baa1/BBB+
State Street Corporation
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,000
Held-to-maturity, Estimated Fair Value	$ 999
Credit Rating Moody’s/ S&P	A1/A
Corporate Debt Securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 59,034
Held-to-maturity, Estimated Fair Value	$ 55,252