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Securities - Carrying Value of Held-to-Maturity Investment Securities (Detail) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]      
Amortized cost   $ 905,496 $ 851,903
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 8,465 8,254  
Carrying value $ 900,614 $ 846,810