FAIR VALUE DISCLOSURES (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 2,540,887	$ 2,531,676	$ 2,016,373
Mortgage servicing rights	67,161	$ 65,263	50,544	$ 57,268
Derivative instruments	12,911		23,479
Loans held for sale	161,600		150,046
Total	2,782,559		2,240,442
Derivative instruments	8,508		19,089
US Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,818,180		1,196,167
U.S. Government Agency Issued Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	167,542		189,741
U.S. Government Agency Issued Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	170,082		207,908
Obligations Of States And Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	352,324		408,537
FHLB and other securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	32,759		14,020
Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	160		810
Recurring [Member] | Level 1 [Member] | FHLB and other securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	160		810
Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for sale	161,600		150,046
Total	2,702,327		2,165,609
Recurring [Member] | Level 2 [Member] | US Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,818,180		1,196,167
Recurring [Member] | Level 2 [Member] | U.S. Government Agency Issued Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	167,542		189,741
Recurring [Member] | Level 2 [Member] | U.S. Government Agency Issued Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	170,082		207,908
Recurring [Member] | Level 2 [Member] | Obligations Of States And Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	352,324		408,537
Recurring [Member] | Level 2 [Member] | FHLB and other securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	32,599		13,210
Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage servicing rights	67,161		50,544
Derivative instruments	12,911		23,479
Total	80,072		74,023
Derivative instruments	$ 8,508		$ 19,089