Employee Benefit Arrangements - Plan Assets, Allocation (Details) - Pension Plan	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	100.00%	100.00%
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	47.20%	45.10%
Equity securities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, minimum	37.00%	37.00%
Equity securities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, minimum	65.00%	65.00%
Large-cap domestic
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	25.30%	23.70%
Small/Mid-cap domestic
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	6.90%	6.40%
International Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	12.50%	11.60%
Other
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	2.50%	3.40%
Debt securities
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	43.70%	44.60%
Debt securities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, minimum	30.00%	30.00%
Debt securities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, minimum	50.00%	50.00%
U.S. Treasuries, short term investments, cash and futures
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	8.00%	6.30%
U.S. government agencies and authorities
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	4.10%	4.20%
U.S. corporate, state and municipalities
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	27.40%	29.70%
Foreign securities
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	4.10%	4.30%
Other fixed maturities
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	0.10%	0.10%
Other investments
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	9.10%	10.30%
Other investments | Minimum
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, minimum	6.00%	6.00%
Other investments | Maximum
Defined Benefit Plan Disclosure [Line Items]
Target allocation range, minimum	14.00%	14.00%
Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	4.20%	4.80%
Real estate
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	4.90%	5.50%