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Investment Securities - Summary of Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Schedule of Investments [Line Items]    
Amortized Cost, Available-for-sale $ 1,970,076 $ 2,078,131
Gross Unrealized Holding Gain, Available-for-sale 3,247 13,282
Gross Unrealized Holding Loss, Available-for-sale (31,731) (10,428)
Fair Value, Available-for-sale $ 1,941,592 $ 2,080,985
Total Percent, Available-for-sale 100.00% 100.00%
Amortized Cost, Held-to-maturity $ 798,284 $ 829,890
Gross Unrealized Holding Gain, Held-to-maturity 1,233 4,272
Gross Unrealized Holding Loss, Held-to-maturity (23,387) (14,947)
Fair Value, Held-to-maturity $ 776,130 $ 819,215
Total Percent, Held-to-maturity 100.00% 100.00%
CMO/REMIC - Residential [Member]    
Schedule of Investments [Line Items]    
Amortized Cost, Available-for-sale $ 259,180 $ 274,634
Gross Unrealized Holding Gain, Available-for-sale 644 1,277
Gross Unrealized Holding Loss, Available-for-sale (4,178) (2,082)
Fair Value, Available-for-sale $ 255,646 $ 273,829
Total Percent, Available-for-sale 13.17% 13.16%
Amortized Cost, Held-to-maturity $ 221,051 $ 225,072
Gross Unrealized Holding Gain, Held-to-maturity 0 0
Gross Unrealized Holding Loss, Held-to-maturity (11,522) (8,641)
Fair Value, Held-to-maturity $ 209,529 $ 216,431
Total Percent, Held-to-maturity 27.69% 27.12%
Government Agency/GSE [Member]    
Schedule of Investments [Line Items]    
Amortized Cost, Held-to-maturity $ 154,194 $ 159,716
Gross Unrealized Holding Gain, Held-to-maturity 473 854
Gross Unrealized Holding Loss, Held-to-maturity (2,453) (2,134)
Fair Value, Held-to-maturity $ 152,214 $ 158,436
Total Percent, Held-to-maturity 19.32% 19.25%
Residential Mortgage-backed Securities [Member]    
Schedule of Investments [Line Items]    
Amortized Cost, Available-for-sale $ 1,655,742 $ 1,747,780
Gross Unrealized Holding Gain, Available-for-sale 2,158 11,231
Gross Unrealized Holding Loss, Available-for-sale (26,560) (8,102)
Fair Value, Available-for-sale $ 1,631,340 $ 1,750,909
Total Percent, Available-for-sale 84.02% 84.14%
Amortized Cost, Held-to-maturity $ 170,573 $ 176,427
Gross Unrealized Holding Gain, Held-to-maturity 0 667
Gross Unrealized Holding Loss, Held-to-maturity (3,191) (382)
Fair Value, Held-to-maturity $ 167,382 $ 176,712
Total Percent, Held-to-maturity 21.36% 21.26%
Municipal Bonds [Member]    
Schedule of Investments [Line Items]    
Amortized Cost, Available-for-sale $ 54,416 $ 54,966
Gross Unrealized Holding Gain, Available-for-sale 445 774
Gross Unrealized Holding Loss, Available-for-sale (993) (244)
Fair Value, Available-for-sale $ 53,868 $ 55,496
Total Percent, Available-for-sale 2.77% 2.66%
Amortized Cost, Held-to-maturity $ 252,466 $ 268,675
Gross Unrealized Holding Gain, Held-to-maturity 760 2,751
Gross Unrealized Holding Loss, Held-to-maturity (6,221) (3,790)
Fair Value, Held-to-maturity $ 247,005 $ 267,636
Total Percent, Held-to-maturity 31.63% 32.37%
Other Securities [Member]    
Schedule of Investments [Line Items]    
Amortized Cost, Available-for-sale $ 738 $ 751
Gross Unrealized Holding Gain, Available-for-sale 0 0
Gross Unrealized Holding Loss, Available-for-sale 0 0
Fair Value, Available-for-sale $ 738 $ 751
Total Percent, Available-for-sale 0.04% 0.04%