Securities - Carrying Value of Held-to-Maturity Investment Securities (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Amortized Cost		$ 990,582	$ 772,060
Net loss on date of transfer from available-for-sale	$ (13,347)	(13,347)
Accretion of net unrealized loss on securities reclassified as held-to-maturity	5,622	5,349
Carrying value	$ 982,857	$ 764,062