DEBT AND OTHER OBLIGATIONS - Debt and Subordinated Debt Securities Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	May 03, 2018
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Debt maturities due in 2019	$ 416,500,000
Debt maturities due thereafter	684,000,000
Outstanding principal	$ 991,363,000	$ 784,610,000
Golden Gate
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Stated interest rate (as a percent)	4.75%
Subordinated debentures
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Outstanding principal	$ 610,000,000	500,000,000
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	65,600,000
Par value of extinguished debt	$ 42,900,000
Stated interest rate (as a percent)	8.45%
Pre-tax gain on extinguishment	$ 1,800,000
Outstanding principal	$ 190,044,000	232,928,000
3.55% Subordinated Debentures (2018), due 2038
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Stated interest rate (as a percent)	3.55%
3.55% Subordinated Debentures (2018), due 2038 | Subordinated debentures
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Outstanding principal	$ 110,000,000
4.30% Senior Notes (2018), due 2028
Future maturities of debt, excluding notes payable to banks, and subordinated debt securities, for the next five years
Stated interest rate (as a percent)	4.30%
Outstanding principal	$ 400,000,000
Discount on notes	1,000,000
Debt issuance costs	$ 3,700,000
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
Outstanding principal	$ 500,000,000	500,000,000
Amount of debt issued	500,000,000.0
Debt issuance costs	$ 4,800,000
6.00% Subordinated Debentures, due 2042
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, due 2042 | 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
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
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 | 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%
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
Interest rate added to the base rate (as a percent)	1.00%
2015 Credit Facility as amended
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
Line of credit, maximum borrowing capacity to be granted upon entity's request			$ 1,500,000,000.0
Facility fee percentage	0.125%
2015 Credit Facility as amended | 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%
2015 Credit Facility as amended | 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%