INVESTMENT OPERATIONS - Summary of Major Categories of Net Investment Income (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended								11 Months Ended	12 Months Ended
	Jan. 31, 2015	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Investment income before other investment expenses										$ 1,657,894	$ 2,080,715	$ 1,971,567
Other investment expenses										24,946	29,127	29,111
Net investment income		$ 529,490	$ 507,914	$ 507,771	$ 506,413	$ 496,150	$ 482,729	$ 488,460	$ 475,117	1,632,948	2,051,588	1,942,456
Fixed maturities
Investment [Line Items]
Investment income before other investment expenses										1,267,900	1,631,565	1,552,999
Equity securities
Investment [Line Items]
Investment income before other investment expenses										40,907	39,806	38,838
Mortgage loans
Investment [Line Items]
Investment income before other investment expenses										252,577	298,387	270,749
Investment real estate
Investment [Line Items]
Investment income before other investment expenses										2,528	2,481	2,153
Short-term investments
Investment [Line Items]
Investment income before other investment expenses										$ 93,982	$ 108,476	$ 106,828
Predecessor
Investment [Line Items]
Investment income before other investment expenses	$ 177,478
Other investment expenses	2,298
Net investment income	175,180
Predecessor | Fixed maturities
Investment [Line Items]
Investment income before other investment expenses	140,104
Predecessor | Equity securities
Investment [Line Items]
Investment income before other investment expenses	2,572
Predecessor | Mortgage loans
Investment [Line Items]
Investment income before other investment expenses	24,977
Predecessor | Investment real estate
Investment [Line Items]
Investment income before other investment expenses	112
Predecessor | Short-term investments
Investment [Line Items]
Investment income before other investment expenses	$ 9,713