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Investment Securities - Summary of Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Schedule of Investments [Line Items]    
Amortized Cost, Available-for-sale $ 2,916,300 $ 2,344,174
Gross Unrealized Holding Gain, Available-for-sale 35,220 55,099
Gross Unrealized Holding Loss, Available-for-sale (26,460) (350)
Investment securities available-for-sale $ 2,925,060 $ 2,398,923
Total Percent, Available-for-sale 100.00% 100.00%
Amortized Cost, Held-to-maturity $ 1,710,938 $ 578,626
Gross Unrealized Holding Gain, Held-to-maturity 18,638 25,703
Gross Unrealized Holding Loss, Held-to-maturity (12,346) (106)
Fair Value, Held-to-maturity $ 1,717,230 $ 604,223
Total Percent, Held-to-maturity 100.00% 100.00%
CMO/REMIC [Member]    
Schedule of Investments [Line Items]    
Amortized Cost, Available-for-sale $ 630,351 $ 457,548
Gross Unrealized Holding Gain, Available-for-sale 2,846 5,515
Gross Unrealized Holding Loss, Available-for-sale (8,030) (249)
Investment securities available-for-sale $ 625,167 $ 462,814
Total Percent, Available-for-sale 21.37% 19.29%
Amortized Cost, Held-to-maturity $ 264,324 $ 145,309
Gross Unrealized Holding Gain, Held-to-maturity 1,642 5,202
Gross Unrealized Holding Loss, Held-to-maturity (1,118) 0
Fair Value, Held-to-maturity $ 264,848 $ 150,511
Total Percent, Held-to-maturity 15.45% 25.11%
Government Agency/GSE [Member]    
Schedule of Investments [Line Items]    
Amortized Cost, Held-to-maturity $ 585,022 $ 98,663
Gross Unrealized Holding Gain, Held-to-maturity 6,785 5,877
Gross Unrealized Holding Loss, Held-to-maturity (8,112) 0
Fair Value, Held-to-maturity $ 583,695 $ 104,540
Total Percent, Held-to-maturity 34.19% 17.05%
Mortgage-backed Securities [Member]    
Schedule of Investments [Line Items]    
Amortized Cost, Available-for-sale $ 2,256,252 $ 1,857,030
Gross Unrealized Holding Gain, Available-for-sale 31,251 48,006
Gross Unrealized Holding Loss, Available-for-sale (18,430) (101)
Investment securities available-for-sale $ 2,269,073 $ 1,904,935
Total Percent, Available-for-sale 77.57% 79.41%
Amortized Cost, Held-to-maturity $ 648,613 $ 146,382
Gross Unrealized Holding Gain, Held-to-maturity 5,450 7,644
Gross Unrealized Holding Loss, Held-to-maturity (1,502) (32)
Fair Value, Held-to-maturity $ 652,561 $ 153,994
Total Percent, Held-to-maturity 37.91% 25.30%
Municipal Bonds [Member]    
Schedule of Investments [Line Items]    
Amortized Cost, Available-for-sale $ 28,697 $ 28,707
Gross Unrealized Holding Gain, Available-for-sale 1,123 1,578
Gross Unrealized Holding Loss, Available-for-sale 0 0
Investment securities available-for-sale $ 29,820 $ 30,285
Total Percent, Available-for-sale 1.02% 1.26%
Amortized Cost, Held-to-maturity $ 212,979 $ 188,272
Gross Unrealized Holding Gain, Held-to-maturity 4,761 6,980
Gross Unrealized Holding Loss, Held-to-maturity (1,614) (74)
Fair Value, Held-to-maturity $ 216,126 $ 195,178
Total Percent, Held-to-maturity 12.45% 32.54%
Other Securities [Member]    
Schedule of Investments [Line Items]    
Amortized Cost, Available-for-sale $ 1,000 $ 889
Gross Unrealized Holding Gain, Available-for-sale 0 0
Gross Unrealized Holding Loss, Available-for-sale 0 0
Investment securities available-for-sale $ 1,000 $ 889
Total Percent, Available-for-sale 0.04% 0.04%