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Investment Securities - Summary of Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Schedule of Investments [Line Items]    
Amortized Cost, Available-for-sale $ 2,255,904 $ 2,255,874
Gross Unrealized Holding Gain, Available-for-sale 23,607 23,264
Gross Unrealized Holding Loss, Available-for-sale (7,808) (8,672)
Fair Value, Available-for-sale $ 2,271,703 $ 2,270,466
Total Percent, Available-for-sale 100.00% 100.00%
Amortized Cost, Held-to-maturity $ 885,057 $ 911,676
Gross Unrealized Holding Gain, Held-to-maturity 1,961 1,138
Gross Unrealized Holding Loss, Held-to-maturity (15,263) (15,440)
Fair Value, Held-to-maturity $ 871,755 $ 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 $ 324,283 $ 345,313
Gross Unrealized Holding Gain, Available-for-sale 3,403 3,361
Gross Unrealized Holding Loss, Available-for-sale (1,242) (1,485)
Fair Value, Available-for-sale $ 326,444 $ 347,189
Total Percent, Available-for-sale 14.37% 15.29%
Amortized Cost, Held-to-maturity $ 238,397 $ 244,419
Gross Unrealized Holding Gain, Held-to-maturity 0 0
Gross Unrealized Holding Loss, Held-to-maturity (7,563) (6,808)
Fair Value, Held-to-maturity $ 230,834 $ 237,611
Total Percent, Held-to-maturity 26.93% 26.81%
Government Agency/GSE [Member]    
Schedule of Investments [Line Items]    
Amortized Cost, Available-for-sale $ 1,749 $ 2,750
Gross Unrealized Holding Gain, Available-for-sale 1 2
Gross Unrealized Holding Loss, Available-for-sale 0 0
Fair Value, Available-for-sale $ 1,750 $ 2,752
Total Percent, Available-for-sale 0.08% 0.12%
Amortized Cost, Held-to-maturity $ 176,281 $ 182,648
Gross Unrealized Holding Gain, Held-to-maturity 751 362
Gross Unrealized Holding Loss, Held-to-maturity (1,620) (1,972)
Fair Value, Held-to-maturity $ 175,412 $ 181,038
Total Percent, Held-to-maturity 19.92% 20.03%
Residential Mortgage-backed Securities [Member]    
Schedule of Investments [Line Items]    
Amortized Cost, Available-for-sale $ 1,848,307 $ 1,822,168
Gross Unrealized Holding Gain, Available-for-sale 19,131 18,812
Gross Unrealized Holding Loss, Available-for-sale (5,685) (6,232)
Fair Value, Available-for-sale $ 1,861,753 $ 1,834,748
Total Percent, Available-for-sale 81.95% 80.81%
Amortized Cost, Held-to-maturity $ 186,480 $ 193,699
Gross Unrealized Holding Gain, Held-to-maturity 0 0
Gross Unrealized Holding Loss, Held-to-maturity (1,528) (1,892)
Fair Value, Held-to-maturity $ 184,952 $ 191,807
Total Percent, Held-to-maturity 21.07% 21.25%
Municipal Bonds [Member]    
Schedule of Investments [Line Items]    
Amortized Cost, Available-for-sale $ 75,886 $ 80,137
Gross Unrealized Holding Gain, Available-for-sale 716 889
Gross Unrealized Holding Loss, Available-for-sale (881) (955)
Fair Value, Available-for-sale $ 75,721 $ 80,071
Total Percent, Available-for-sale 3.33% 3.53%
Amortized Cost, Held-to-maturity $ 283,899 $ 290,910
Gross Unrealized Holding Gain, Held-to-maturity 1,210 776
Gross Unrealized Holding Loss, Held-to-maturity (4,552) (4,768)
Fair Value, Held-to-maturity $ 280,557 $ 286,918
Total Percent, Held-to-maturity 32.08% 31.91%
Other Securities [Member]    
Schedule of Investments [Line Items]    
Amortized Cost, Available-for-sale $ 5,679 $ 5,506
Gross Unrealized Holding Gain, Available-for-sale 356 200
Gross Unrealized Holding Loss, Available-for-sale 0 0
Fair Value, Available-for-sale $ 6,035 $ 5,706
Total Percent, Available-for-sale 0.27% 0.25%