Employee Benefit Arrangements - Fair Value of Plan Assets (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) fund_asset	Dec. 31, 2016 USD ($) fund_asset	Dec. 31, 2015 USD ($)
Level 3 | Other fixed maturities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	$ 0	$ 0
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	1,764,000,000	1,463,000,000	$ 1,395,000,000
Net, total pension assets, net asset value	560,000,000	460,000,000
Pension Plan | Debt securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	772,000,000	652,000,000
Net, total pension assets, net asset value	136,000,000	90,000,000
Pension Plan | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	7,000,000	2,000,000
Pension Plan | Short-term investments fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	136,000,000	90,000,000
Net, total pension assets, net asset value	136,000,000	90,000,000
Pension Plan | U.S. Government securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	73,000,000	61,000,000
Pension Plan | U.S. corporate, state and municipalities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	483,000,000	435,000,000
Pension Plan | Foreign securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	72,000,000	63,000,000
Pension Plan | Other fixed maturities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	1,000,000	1,000,000
Pension Plan | Equity securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	830,000,000	660,000,000
Net, total pension assets, net asset value	263,000,000	219,000,000
Pension Plan | Large-cap domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	446,000,000	347,000,000
Pension Plan | Small/Mid-cap domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	121,000,000	94,000,000
Pension Plan | International Commingled Funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	220,000,000	170,000,000
Net, total pension assets, net asset value	$ 220,000,000	$ 170,000,000
Number of Assets In Fund | fund_asset	2	2
Pension Plan | International Commingled Funds | Baillie Gifford Funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	$ 111,000,000	$ 84,000,000
Unfunded commitments	0	0
Pension Plan | International Commingled Funds | Silchester
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	$ 109,000,000	$ 86,000,000
Number of business days prior to month-end clients must submit redemption request	6 days	6 days
Unfunded commitments	$ 0	$ 0
Pension Plan | Equity Securities, Limited Partnerships [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Number of Assets In Fund | fund_asset	2	2
Pension Plan | Limited partnerships
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	$ 43,000,000	$ 49,000,000
Net, total pension assets, net asset value	43,000,000	49,000,000
Pension Plan | Pantheon Europe | Patheon Europe
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	6,000,000	7,000,000
Unfunded commitments	1,000,000	1,000,000
Pension Plan | Pantheon USA | Patheon USA
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	37,000,000	42,000,000
Unfunded commitments	5,000,000	5,000,000
Pension Plan | Other investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	162,000,000	151,000,000
Net, total pension assets, net asset value	161,000,000	151,000,000
Pension Plan | Real estate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	86,000,000	81,000,000
Net, total pension assets, net asset value	86,000,000	81,000,000
Pension Plan | Real estate | UBS Trumbull Property Fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	$ 86,000,000	$ 81,000,000
Number of business days prior to month-end clients must submit redemption request	60 days	60 days
Real return performance objective, rate of return	5.00%	5.00%
Pension Plan | Real estate | UBS Trumbull Property Fund | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Real return performance objective, rate of return, determination period	3 years	3 years
Pension Plan | Real estate | UBS Trumbull Property Fund | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Real return performance objective, rate of return, determination period	5 years	5 years
Pension Plan | Limited partnerships
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	$ 75,000,000	$ 70,000,000
Net, total pension assets, net asset value	75,000,000	70,000,000
Pension Plan | Limited partnerships | Magnitude Institutional, Ltd.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	75,000,000	70,000,000
Pension Plan | Other derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	1,000,000	0
Pension Plan | Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	648,000,000	504,000,000
Pension Plan | Level 1 | Debt securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	80,000,000	63,000,000
Pension Plan | Level 1 | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	7,000,000	2,000,000
Pension Plan | Level 1 | Short-term investments fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 1 | U.S. Government securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	73,000,000	61,000,000
Pension Plan | Level 1 | U.S. corporate, state and municipalities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 1 | Foreign securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 1 | Other fixed maturities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 1 | Equity securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	567,000,000	441,000,000
Pension Plan | Level 1 | Large-cap domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	446,000,000	347,000,000
Pension Plan | Level 1 | Small/Mid-cap domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	121,000,000	94,000,000
Pension Plan | Level 1 | International Commingled Funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 1 | Limited partnerships
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 1 | Other investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	1,000,000	0
Pension Plan | Level 1 | Real estate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 1 | Limited partnerships
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 1 | Other derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	1,000,000	0
Pension Plan | Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	549,000,000	499,000,000
Pension Plan | Level 2 | Debt securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	549,000,000	499,000,000
Pension Plan | Level 2 | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 2 | Short-term investments fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 2 | U.S. Government securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 2 | U.S. corporate, state and municipalities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	476,000,000	435,000,000
Pension Plan | Level 2 | Foreign securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	72,000,000	63,000,000
Pension Plan | Level 2 | Other fixed maturities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	1,000,000	1,000,000
Pension Plan | Level 2 | Equity securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 2 | Large-cap domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 2 | Small/Mid-cap domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 2 | International Commingled Funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 2 | Limited partnerships
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 2 | Other investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 2 | Real estate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 2 | Limited partnerships
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 2 | Other derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	7,000,000	0
Pension Plan | Level 3 | Debt securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	7,000,000	0
Pension Plan | Level 3 | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 3 | Short-term investments fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 3 | U.S. Government securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 3 | U.S. corporate, state and municipalities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	7,000,000	0
Pension Plan | Level 3 | Foreign securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 3 | Equity securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 3 | Large-cap domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 3 | Small/Mid-cap domestic
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 3 | International Commingled Funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 3 | Limited partnerships
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 3 | Other investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 3 | Real estate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 3 | Limited partnerships
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	0	0
Pension Plan | Level 3 | Other derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net, total pension assets, fair value	$ 0	$ 0