INVESTMENTS - Other Invested Assets (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Other invested assets
Number of investments in low income housing tax credit partnerships | item	3	3
Other Investments	$ 33,808	$ 33,808	$ 24,115
Qualified Affordable Housing Project Investments, Commitment	3,100	3,100
Investment in FHLBC	1,000	1,000	1,600
Available-for-sale Securities Pledged as Collateral	18,900	18,900
Proceeds from Federal Home Loan Bank Borrowings	5,500
Repayments of Federal Home Loan Bank Borrowings	5,500
Federal Home Loan Bank Borrowings, Fair Value Disclosure	0	0
Investment in Real Estate Fund
Other invested assets
Other Investments	$ 2,500	$ 2,500	5,000
Investment In Low Income Housing Tax Credit Partnership Net Of Amortization Member
Other invested assets
Number of investments in low income housing tax credit partnerships | item	3	3
Other Investments	$ 15,500	$ 15,500	17,500
Qualified Affordable Housing Project Investments, Commitment	3,100	3,100
Total tax benefit on investments in housing tax credit partnership		2,400	$ 1,900	$ 1,100
Investment in BDC
Other invested assets
Other Investments	7,300	7,300
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	17,700	17,700
Investment In Global Credit Fund
Other invested assets
Other Investments	7,500	7,500
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	$ 7,200	$ 7,200