DEBT AND OTHER OBLIGATIONS - Debt and Subordinated Debt Securities Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Debt maturities due in 2018	$ 150,500,000
Debt maturities due in 2019	435,800,000
Debt maturities due thereafter	357,000,000
Total debt	945,052,000	$ 1,163,285,000
Credit Facility
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Total debt	$ 0	170,000,000
6.40% Senior Notes (2007), due 2018
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Stated interest rate (as a percent)	6.40%
Total debt	$ 150,518,000	156,663,000
7.375% Senior Notes (2009), due 2019
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Stated interest rate (as a percent)	7.375%
Total debt	$ 435,806,000	454,688,000
8.45% Senior Notes (2009), due 2039
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Redemption of subordinated debt	21,600,000
Par value of extinguished debt	$ 14,000,000	$ 53,100,000
Stated interest rate (as a percent)	8.45%	8.45%
Pre-tax gain on extinguishment	$ 2,000,000	$ 9,800,000
Total debt	$ 357,046,000	381,934,000
5.35% Subordinated Debentures (2017), due 2052
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Stated interest rate (as a percent)	5.35%
5.35% Subordinated Debentures (2017), due 2052 | Subordinated debentures
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Debt issuance costs	$ 4,800,000
Amount of debt issued	$ 500,000,000.0
6.125% Subordinated Debentures (2004), due 2034, callable 2009
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Repayment of debt		82,700,000
6.00% Subordinated Debentures (2012), due 2042, callable 2017
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Stated interest rate (as a percent)	6.00%
6.00% Subordinated Debentures (2012), due 2042, callable 2017 | Subordinated debentures
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Repayment of debt		150,000,000
6.25% Subordinated Debentures (2012), due 2042, callable 2017
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Stated interest rate (as a percent)	6.25%
6.25% Subordinated Debentures (2012), due 2042, callable 2017 | Subordinated debentures
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Repayment of debt		$ 287,500,000
Credit Facility | Federal Funds Rate
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Interest rate added to the base rate (as a percent)	0.50%
Credit Facility | LIBOR One-Month Rate
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Interest rate added to the base rate (as a percent)	1.00%
2015 Credit Facility
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Line of credit, maximum borrowing capacity	$ 1,000,000,000.0
Line of credit, maximum borrowing capacity to be granted upon entity's request	$ 1,250,000,000.00
Facility fee percentage	0.125%
2015 Credit Facility | Prime Rate
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Base of floating rate interest rate payments	prime rate
2015 Credit Facility | LIBOR One-Month Rate
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Base of floating rate interest rate payments	one-month LIBOR