INVESTMENT SECURITIES (Amortized Cost and Estimated Fair Value of Investment Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Held to Maturity, Amortized Cost
Mortgage-backed securities	$ 197,672
Held to Maturity, Estimated Fair Value
Mortgage-backed securities	195,714
Available for Sale, Amortized Cost
Due in one year or less	10,193
Due after one year through five years	152,586
Due after five years through ten years	52,282
Due after ten years	85,035
Total	1,345,474
Available for Sale, Estimated Fair Value
Due in one year or less	10,228
Due after one year through five years	154,716
Due after five years through ten years	53,293
Due after ten years	85,925
Available-for-sale, at fair value	1,350,105	$ 1,243,847
Residential - U.S. Government-sponsored entities
Held to Maturity, Amortized Cost
Mortgage-backed securities	105,659
Held to Maturity, Estimated Fair Value
Mortgage-backed securities	103,954
Available for Sale, Amortized Cost
Mortgage-backed securities	822,342
Available for Sale, Estimated Fair Value
Mortgage-backed securities	819,086
Available-for-sale, at fair value	819,086	769,986
Commercial - U.S. Government agencies and sponsored entities
Available for Sale, Amortized Cost
Mortgage-backed securities	40,385
Available for Sale, Estimated Fair Value
Mortgage-backed securities	40,228
Available-for-sale, at fair value	40,228
Commercial - U.S. Government-sponsored entities
Held to Maturity, Amortized Cost
Mortgage-backed securities	92,013
Held to Maturity, Estimated Fair Value
Mortgage-backed securities	91,760
Residential - Non-government agencies
Available for Sale, Amortized Cost
Mortgage-backed securities	46,881
Available for Sale, Estimated Fair Value
Mortgage-backed securities	47,464
Available-for-sale, at fair value	47,464	51,547
Commercial - Non-government agencies
Available for Sale, Amortized Cost
Mortgage-backed securities	135,105
Available for Sale, Estimated Fair Value
Mortgage-backed securities	138,371
Available-for-sale, at fair value	138,371	137,224
Other
Available for Sale, Amortized Cost
Other	665
Available for Sale, Estimated Fair Value
Other	794
Available-for-sale, at fair value	$ 794	$ 660