NOTES PAYABLE - DIRECT CORPORATE OBLIGATIONS (NEW REVOLVING CREDIT AGREEMENT) (NARRATIVE) (Details) - Line of Credit [Member] - USD ($)	May 19, 2015	Dec. 31, 2016
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Initial drawing amount	$ 100,000,000
Remaining borrowing capacity	50,000,000
Maximum borrowing capacity	150,000,000
Potential additional borrowing capacity	$ 50,000,000.0
Interest rate on amounts outstanding at period end		2.77%
Fronting fee as a percent of aggregate face amount of letters of credit outstanding	0.125%
Debt covenant, required minimum debt to total capitalization ratio	30.00%
Debt covenant, actual debt to total capitalization ratio at period end		19.40%
Debt covenant, minimum required aggregate total adjusted capital to company action level risk-based capital ratio	250.00%
Debt covenant, actual aggregate total adjusted capital to company action level risk-based capital ratio at period end		459.00%
Debt covenant, minimum required consolidated net worth, component one, amount	$ 2,674,000,000
Debt covenant, minimum required consolidated net worth, component two, as a percent of net equity proceeds received from issuance and sale of equity interests	50.00%
Debt covenant, actual consolidated net worth at period end		$ 3,864,500,000
Debt covenant, required minimum consolidated net worth, amount		$ 2,680,800,000
Revolving Credit Facility [Member] | Federal Funds Rate [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	0.50%
Revolving Credit Facility [Member] | One-Month Eurodollar [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.00%
Revolving Credit Facility [Member] | Eurodollar [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	2.00%
Bridge Loan [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 5,000,000.0
Letter of Credit [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 5,000,000.0