DEBT AND OTHER OBLIGATIONS - Non-Recourse Funding Obligations (Details) - USD ($)				3 Months Ended	12 Months Ended
	Dec. 31, 2019	Jan. 15, 2016	Oct. 10, 2012	Jun. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	May 03, 2018
Debt Instrument [Line Items]
Estimated initial ceding allowance receivable	$ 76,400,000				$ 76,400,000
Non-recourse funding obligations	3,082,753,000				3,082,753,000	$ 2,888,329,000
Repurchase of non-recourse funding obligations, at a discount					38,000,000.0
Payments under agreement					195,000,000	(63,890,000)	$ (16,070,000)
2015 Credit Facility
Debt Instrument [Line Items]
Principal amount	1,000,000,000.0				1,000,000,000.0
Maximum borrowing capacity	1,250,000,000				$ 1,250,000,000
Facility fee rate					0.125%
Amended Line Of Credit
Debt Instrument [Line Items]
Principal amount	$ 1,000,000,000.0				$ 1,000,000,000.0
Maximum borrowing capacity								$ 1,500,000,000
Facility fee rate					0.125%
Financing Agreement With Golden Gate And Syndicate Of Risk Takers
Debt Instrument [Line Items]
Term of financing agreement		18 years
Financing capacity under the agreement		$ 2,188,000,000
Subordinated Debentures 3.55 Percent Due 2038
Debt Instrument [Line Items]
Interest rate	3.55%				3.55%
Golden Gate
Debt Instrument [Line Items]
Estimated transfer of policyholder liabilities	$ 76,400,000
Initial premiums transferred	76,400,000				$ 76,400,000
Golden Gate Captive Insurance Company (Golden Gate)
Debt Instrument [Line Items]
Non-recourse funding obligations	2,028,000,000				$ 2,028,000,000	$ 1,883,000,000
Fixed annual rate					4.70%	4.75%
Payments made under agreement					$ 0
Golden Gate II Captive Insurance Company (Golden Gate II)
Debt Instrument [Line Items]
Non-recourse funding obligations	274,955,000				$ 274,955,000	$ 273,535,000
Fixed annual rate					5.06%	4.24%
Outstanding non-recourse funding obligations	575,000,000				$ 575,000,000
Non-recourse funding held by external parties	20,600,000				20,600,000
Non-recourse funding held by nonconsolidated affiliates	309,300,000				309,300,000
Non-recourse funding held by consolidated subsidiaries	245,100,000				245,100,000
Payments made under agreement				$ 1,000,000.0	1,000,000.0
Collateralized obligations under supper agreements	4,900,000				4,900,000
Golden Gate V Vermont Captive Insurance Company And Red Mountain L L C
Debt Instrument [Line Items]
Term of transaction			20 years
Maximum financing under transaction			$ 945,000,000
Red Mountain
Debt Instrument [Line Items]
Initial principal amount			275,000,000
Future maximum financing amount			$ 945,000,000
Principal balance of note	$ 720,000,000				720,000,000
Payments under agreement					$ 0
Golden Gate V Vermont Captive Insurance Company (Golden Gate V)
Debt Instrument [Line Items]
Interest rate	6.25%				6.25%
Non-recourse funding obligations	$ 777,527,000				$ 777,527,000	$ 729,454,000
Fixed annual rate					5.12%	5.12%
Future scheduled capital contributions					$ 107,300,000
Outstanding non-recourse funding obligation	720,000,000				720,000,000
Maximum principal amount	945,000,000				945,000,000
Subordinated Debt | Subordinated Debentures 3.55 Percent Due 2038
Debt Instrument [Line Items]
Outstanding principal	110,000,000.0				110,000,000.0
Surplus Notes | Golden Gate | Steel City Notes
Debt Instrument [Line Items]
Amount of debt issued	$ 2,028,000,000.000	2,188,000,000			$ 2,028,000,000.000
Surplus Notes | Steel City | Golden Gate Surplus Notes
Debt Instrument [Line Items]
Amount of debt issued		$ 2,188,000,000
Base Rate | 2015 Credit Facility
Debt Instrument [Line Items]
Variable rate					0.50%
Base Rate | Amended Line Of Credit
Debt Instrument [Line Items]
Variable rate					0.50%
LIBOR | 2015 Credit Facility
Debt Instrument [Line Items]
Variable rate					1.00%
LIBOR | Amended Line Of Credit
Debt Instrument [Line Items]
Variable rate					1.00%