DEBT AND OTHER OBLIGATIONS - Summary of Debt and Subordinated Debt Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Outstanding Principal	$ 991,363	$ 784,610
Total debt	1,101,827	945,052
Subordinated debt	605,426	495,289
6.40% Senior Notes (2007), due 2018
Debt Instrument [Line Items]
Outstanding Principal	0	150,000
Total debt	$ 0	150,518
Stated interest rate (as a percent)	6.40%
7.375% Senior Notes (2009), due 2019
Debt Instrument [Line Items]
Outstanding Principal	$ 400,000	400,000
Total debt	$ 416,469	435,806
Stated interest rate (as a percent)	7.375%
8.45% Senior Notes (2009), due 2039
Debt Instrument [Line Items]
Outstanding Principal	$ 190,044	232,928
Total debt	$ 288,547	357,046
Stated interest rate (as a percent)	8.45%
4.30% Senior Notes (2018), due 2028
Debt Instrument [Line Items]
Outstanding Principal	$ 400,000
Total debt	$ 395,492	0
Stated interest rate (as a percent)	4.30%
5.35% Subordinated Debentures (2017), due 2052
Debt Instrument [Line Items]
Stated interest rate (as a percent)	5.35%
Capital lease obligation
Debt Instrument [Line Items]
Outstanding Principal	$ 1,319	1,682
Total debt	1,319	1,682
Subordinated debentures
Debt Instrument [Line Items]
Outstanding Principal	610,000	500,000
Subordinated debt	605,426	495,289
Subordinated debentures | 5.35% Subordinated Debentures (2017), due 2052
Debt Instrument [Line Items]
Outstanding Principal	500,000	500,000
Subordinated debt	495,426	495,289
Subordinated debentures | 3.55% Subordinated Funding Obligations (2018), due 2038
Debt Instrument [Line Items]
Outstanding Principal	55,000	0
Subordinated debt	$ 55,000	0
Stated interest rate (as a percent)	3.55%
Subordinated debentures | 3.55% Subordinated Funding Obligations (2018), due 2038
Debt Instrument [Line Items]
Outstanding Principal	$ 55,000	0
Subordinated debt	$ 55,000	0
Stated interest rate (as a percent)	3.55%
Credit Facility
Debt Instrument [Line Items]
Outstanding Principal	$ 0	0
Total debt	$ 0	$ 0