INVESTMENT SECURITIES (Amortized Cost and Estimated Fair Value of Investment Securities) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Held to Maturity, Amortized Cost
Mortgage-backed securities			$ 254,719
Held to Maturity, Estimated Fair Value
Mortgage-backed securities			254,540
Available for Sale, Amortized Cost
Due in one year or less			7,559
Due after one year through five years			98,871
Due after five years through ten years			82,352
Due after ten years			102,564
Total			1,255,100
Available for Sale, Estimated Fair Value
Due in one year or less			7,690
Due after one year through five years			100,540
Due after five years through ten years			83,760
Due after ten years			103,141
Available for sale, at fair value			1,272,382		$ 1,229,018
Available for sale
Average yield held to maturity	1.35%
Weighted average life of held to maturity	4 years 4 months 24 days
Investment in held to maturity investments	$ 117,500
Investment in available-for-sale securities	$ 137,400
Average net yield of available for sale securities sold	2.68%
Weighted average life of available for sale securities sold	7 years 6 months
Gross proceeds from sale of available for sale investment securities		$ 162,500	117,496	$ 162,470
Gross realized gains on the sales of the available for sale investment securities		900
Gross realized losses on the sales of the available for sale investment securities	$ 1,900	$ 700
Investment securities pledged to party with no right to sell or repledge the collateral			1,010,000		900,500
Residential - U.S. Government sponsored entities
Held to Maturity, Amortized Cost
Mortgage-backed securities			158,622
Held to Maturity, Estimated Fair Value
Mortgage-backed securities			157,371
Available for Sale, Amortized Cost
Mortgage-backed securities			760,884
Available for Sale, Estimated Fair Value
Mortgage-backed securities			768,586
Available for sale, at fair value			768,586		751,558
Commercial - U.S. Government sponsored entities
Held to Maturity, Amortized Cost
Mortgage-backed securities			96,097
Held to Maturity, Estimated Fair Value
Mortgage-backed securities			97,169
Residential - Non-government agencies
Available for Sale, Amortized Cost
Mortgage-backed securities			66,560
Available for Sale, Estimated Fair Value
Mortgage-backed securities			69,012
Available for sale, at fair value			69,012		46,693
Commercial - Non-government agencies
Available for Sale, Amortized Cost
Mortgage-backed securities			135,487
Available for Sale, Estimated Fair Value
Mortgage-backed securities			138,761
Available for sale, at fair value			138,761		137,641
Other
Available for Sale, Amortized Cost
Other			823
Available for Sale, Estimated Fair Value
Other			892
Available for sale, at fair value			$ 892		$ 877