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Investment Securities - Summary of Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Schedule of Investments [Line Items]    
Amortized Cost, Available-for-sale $ 2,078,131 $ 2,255,874
Gross Unrealized Holding Gain, Available-for-sale 13,282 23,264
Gross Unrealized Holding Loss, Available-for-sale (10,428) (8,672)
Fair Value, Available-for-sale $ 2,080,985 $ 2,270,466
Total Percent, Available-for-sale 100.00% 100.00%
Amortized Cost, Held-to-maturity $ 829,890 $ 911,676
Gross Unrealized Holding Gain, Held-to-maturity 4,272 1,138
Gross Unrealized Holding Loss, Held-to-maturity (14,947) (15,440)
Fair Value, Held-to-maturity $ 819,215 $ 897,374
Total Percent, Held-to-maturity 100.00% 100.00%
CMO/REMIC - Residential [Member]    
Schedule of Investments [Line Items]    
Amortized Cost, Available-for-sale $ 274,634 $ 345,313
Gross Unrealized Holding Gain, Available-for-sale 1,277 3,361
Gross Unrealized Holding Loss, Available-for-sale (2,082) (1,485)
Fair Value, Available-for-sale $ 273,829 $ 347,189
Total Percent, Available-for-sale 13.16% 15.29%
Amortized Cost, Held-to-maturity $ 225,072 $ 244,419
Gross Unrealized Holding Gain, Held-to-maturity 0 0
Gross Unrealized Holding Loss, Held-to-maturity (8,641) (6,808)
Fair Value, Held-to-maturity $ 216,431 $ 237,611
Total Percent, Held-to-maturity 27.12% 26.81%
Government Agency/GSE [Member]    
Schedule of Investments [Line Items]    
Amortized Cost, Available-for-sale   $ 2,750
Gross Unrealized Holding Gain, Available-for-sale   2
Gross Unrealized Holding Loss, Available-for-sale   0
Fair Value, Available-for-sale   $ 2,752
Total Percent, Available-for-sale   0.12%
Amortized Cost, Held-to-maturity $ 159,716 $ 182,648
Gross Unrealized Holding Gain, Held-to-maturity 854 362
Gross Unrealized Holding Loss, Held-to-maturity (2,134) (1,972)
Fair Value, Held-to-maturity $ 158,436 $ 181,038
Total Percent, Held-to-maturity 19.25% 20.03%
Residential Mortgage-backed Securities [Member]    
Schedule of Investments [Line Items]    
Amortized Cost, Available-for-sale $ 1,747,780 $ 1,822,168
Gross Unrealized Holding Gain, Available-for-sale 11,231 18,812
Gross Unrealized Holding Loss, Available-for-sale (8,102) (6,232)
Fair Value, Available-for-sale $ 1,750,909 $ 1,834,748
Total Percent, Available-for-sale 84.14% 80.81%
Amortized Cost, Held-to-maturity $ 176,427 $ 193,699
Gross Unrealized Holding Gain, Held-to-maturity 667 0
Gross Unrealized Holding Loss, Held-to-maturity (382) (1,892)
Fair Value, Held-to-maturity $ 176,712 $ 191,807
Total Percent, Held-to-maturity 21.26% 21.25%
Municipal Bonds [Member]    
Schedule of Investments [Line Items]    
Amortized Cost, Available-for-sale $ 54,966 $ 80,137
Gross Unrealized Holding Gain, Available-for-sale 774 889
Gross Unrealized Holding Loss, Available-for-sale (244) (955)
Fair Value, Available-for-sale $ 55,496 $ 80,071
Total Percent, Available-for-sale 2.66% 3.53%
Amortized Cost, Held-to-maturity $ 268,675 $ 290,910
Gross Unrealized Holding Gain, Held-to-maturity 2,751 776
Gross Unrealized Holding Loss, Held-to-maturity (3,790) (4,768)
Fair Value, Held-to-maturity $ 267,636 $ 286,918
Total Percent, Held-to-maturity 32.37% 31.91%
Other Securities [Member]    
Schedule of Investments [Line Items]    
Amortized Cost, Available-for-sale $ 751 $ 5,506
Gross Unrealized Holding Gain, Available-for-sale 0 200
Gross Unrealized Holding Loss, Available-for-sale 0 0
Fair Value, Available-for-sale $ 751 $ 5,706
Total Percent, Available-for-sale 0.04% 0.25%