Schedule II. Condensed Financial Information of Registrant (Parent Company) (Schedule of Condensed Financial Information of Registrant, Statements of Cash Flows) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	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, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flows from Operating Activities
Net income	$ 223	$ 144	$ 272	$ 260	$ 241	$ 343	$ 209	$ 66	$ 899	$ 859	$ 479
Adjustments to reconcile net income to net cash flows provided by operating activities:
Net realized investment losses (gains)									(93)	(50)	67
Other, net									38	29	(2)
Total adjustments									355	557	908
Net cash flows provided by operating activities									1,254	1,416	1,387
Cash Flows from Investing Activities
Change in short term investments									(6)	258	34
Other, net									20	2	21
Net cash flows used by investing activities									(424)	(846)	(372)
Cash Flows from Financing Activities
Dividends paid to common stockholders									(842)	(813)	(811)
Proceeds from the issuance of debt									496	498	0
Repayment of debt									(391)	(358)	0
Other, net									(18)	0	4
Net cash flows used by financing activities									(755)	(673)	(807)
Net change in cash									84	(116)	197
Cash, beginning of year				271				387	271	387	190
Cash, end of year	355				271				355	271	387
Parent Company
Cash Flows from Operating Activities
Net income									899	859	479
Adjustments to reconcile net income to net cash flows provided by operating activities:
Equity in net income of subsidiaries									(1,032)	(978)	(594)
Dividends received from subsidiaries									955	765	900
Net realized investment losses (gains)									42	7	(5)
Other, net									36	21	4
Total adjustments									1	(185)	305
Net cash flows provided by operating activities									900	674	784
Cash Flows from Investing Activities
Change in short term investments									(146)	(9)	21
Other, net									0	4	7
Net cash flows used by investing activities									(146)	(5)	28
Cash Flows from Financing Activities
Dividends paid to common stockholders									(842)	(813)	(811)
Proceeds from the issuance of debt									496	498	0
Repayment of debt									(391)	(358)	0
Stock options exercised									0	0	1
Other, net									(17)	1	1
Net cash flows used by financing activities									(754)	(672)	(809)
Net change in cash									0	(3)	3
Cash, beginning of year				$ 1				$ 4	1	4	1
Cash, end of year	$ 1				$ 1				$ 1	$ 1	$ 4