Schedule II - Condensed Financial Information, Statements of Cash Flows (Details) - USD ($) $ in Millions		1 Months Ended	3 Months Ended								12 Months Ended
		Apr. 30, 2015	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
Shareholders' net income			$ 266	$ 560	$ 813	$ 598	$ 382	$ 456	$ 510	$ 519	$ 2,237	$ 1,867	$ 2,094
Adjustments to reconcile shareholders' net income to net cash provided by (used in) operating activities:
Other liabilities											639	124	423
Debt extinguishment costs		$ 100		$ 321							321	0	100
Other, net	[1]										36	(10)	71
Net cash provided by (used in) operating activities	[1],[2]										4,086	4,026	2,933
Cash Flows from Investing Activities
Short term investments purchased											(1,065)	(1,698)	(1,183)
Other, net											0	(101)	0
Net cash provided by (used in) investing activities	[2]										(1,703)	(2,574)	(1,736)
Cash Flows from Financing Activities
Net change in short-term debt											80	(148)	(21)
Payments for debt extinguishment											(313)	0	(87)
Repayment of long-term debt						(250)					(1,250)	0	(851)
Net proceeds on issuance of long-term debt											1,581	0	894
Issuance of common stock											131	36	154
Repurchase of common stock											(2,725)	(139)	(671)
Other, net	[1]										(22)	(72)	(97)
Net cash provided by (used in) financing activities	[1]										(2,651)	(225)	(609)
Net increase (decrease) in cash and cash equivalents											(213)	1,217	548
Cash and cash equivalents, January 1,						3,185				1,968	3,185	1,968	1,420
Cash and cash equivalents, December 31,			2,972				3,185				2,972	3,185	1,968
Accounting Standards Update 2016-09 [Member]
Adjustments to reconcile shareholders' net income to net cash provided by (used in) operating activities:
Other, net											61	72
Net cash provided by (used in) operating activities											61	72
Cash Flows from Financing Activities
Other, net											(61)	(72)
Net cash provided by (used in) financing activities											(61)	(72)
Accounting Standards Update 2016-09 [Member] | Restatement Adjustment [Member]
Adjustments to reconcile shareholders' net income to net cash provided by (used in) operating activities:
Other, net													79
Net cash provided by (used in) operating activities													79
Cash Flows from Financing Activities
Other, net													(79)
Net cash provided by (used in) financing activities													(79)
Parent Company [Member]
Cash Flows from Operating Activities
Shareholders' net income											2,237	1,867	2,094
Adjustments to reconcile shareholders' net income to net cash provided by (used in) operating activities:
Equity in income from subsidiaries											(2,823)	(2,249)	(2,454)
Dividends received from subsidiaries											758	580	880
Other liabilities											(224)	(9)	112
Debt extinguishment costs											321	0	100
Other, net	[1]										333	187	112
Net cash provided by (used in) operating activities	[1]										602	376	844
Cash Flows from Investing Activities
Short term investments purchased											(6)	(3)	(54)
Other, net											(11)	(8)	(14)
Net cash provided by (used in) investing activities											(17)	(11)	(68)
Cash Flows from Financing Activities
Net change in amounts due to / from affiliates											1,955	(78)	(161)
Net change in short-term debt											100	(100)	0
Payments for debt extinguishment											(313)	0	(87)
Repayment of long-term debt											(1,250)	0	(851)
Net proceeds on issuance of long-term debt											1,581	0	894
Issuance of common stock											131	36	154
Common dividends paid											(10)	(10)	(10)
Repurchase of common stock											(2,725)	(139)	(671)
Tax withholding on stock compensation											(61)	(72)	(79)
Other, net	[1]										(2)	0	0
Net cash provided by (used in) financing activities	[1]										(594)	(363)	(811)
Net increase (decrease) in cash and cash equivalents											(9)	2	(35)
Cash and cash equivalents, January 1,						$ 18				$ 16	18	16	51
Cash and cash equivalents, December 31,			$ 9				$ 18				9	18	16
Parent Company [Member] | Accounting Standards Update 2016-09 [Member]
Adjustments to reconcile shareholders' net income to net cash provided by (used in) operating activities:
Other, net											61	72
Net cash provided by (used in) operating activities											61	72
Cash Flows from Financing Activities
Tax withholding on stock compensation											(61)	(72)
Net cash provided by (used in) financing activities											$ (61)	$ (72)
Parent Company [Member] | Accounting Standards Update 2016-09 [Member] | Restatement Adjustment [Member]
Adjustments to reconcile shareholders' net income to net cash provided by (used in) operating activities:
Other, net													79
Net cash provided by (used in) operating activities													79
Cash Flows from Financing Activities
Tax withholding on stock compensation													(79)
Net cash provided by (used in) financing activities													$ (79)

[1] As required in adopting Accounting Standard Update ("ASU") 2016-09 in 2016, the Company retrospectively reclassified $79 million of cash payments from operating to financing activities in 2015. These payments were related to employee tax obligations associated with stock compensation. The comparable amounts reported in financing activities were $61 million in 2017 and $72 million in 2016.

[2] As required in adopting ASU 2016-15 in 2016, the Company retrospectively reclassified $137 million of cash distributions of earnings from partnership investments from investing to operating activities in 2015. The comparable amounts reported in operating activities were $161 million in 2017 and $144 million in 2016.