Consolidating Financial Statements (Details) - Consolidating Statement of Cash Flows (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Consolidating Statement of Cash Flows
Net income	$ 524	$ 595	$ 1,193	$ 1,212
Net adjustments to reconcile net income to net cash provided by operating activities			505	542
Net cash provided by operating activities			1,698	1,754
Proceeds from maturities of fixed maturities			3,657	4,300
Proceeds from sales of investments:
Fixed maturities			2,607	563
Equity securities			92
Equity securities				200
Real estate investments				20
Other investments			189	233
Purchases of investments:
Fixed maturities			(7,952)	(5,673)
Equity securities			(60)
Equity securities				(166)
Real estate investments			(44)	(26)
Other investments			(275)	(259)
Net sales (purchases) of short-term securities			1,202	(424)
Securities transactions in course of settlement			279	170
Other investing activities			(152)	(128)
Net cash used in investing activities			(457)	(1,190)
Treasury stock acquired — share repurchase authorization			(700)	(700)
Treasury stock acquired — net employee share-based compensation			(51)	(61)
Dividends paid to shareholders			(404)	(389)
Payment of debt			(600)	(207)
Issuance of debt			491	689
Issuance of common stock — employee share options			98	118
Net cash used in financing activities			(1,166)	(550)
Effect of exchange rate changes on cash			(4)	7
Net increase in cash			71	21
Cash at beginning of year			344	307
Cash at end of period	415	328	415	328
Income taxes paid (received)			238	323
Interest paid			175	178
Reportable Legal Entities [Member] | TPC [Member]
Consolidating Statement of Cash Flows
Net income	458	438	1,009	946
Net adjustments to reconcile net income to net cash provided by operating activities			323	341
Net cash provided by operating activities			1,332	1,287
Proceeds from maturities of fixed maturities			2,730	3,211
Proceeds from sales of investments:
Fixed maturities			1,847	285
Equity securities			33
Equity securities				4
Other investments			133	176
Purchases of investments:
Fixed maturities			(5,762)	(4,160)
Equity securities			(2)
Equity securities				(3)
Other investments			(232)	(202)
Net sales (purchases) of short-term securities			986	383
Securities transactions in course of settlement			268	105
Other investing activities			(148)	(134)
Net cash used in investing activities			(147)	(335)
Dividends paid to parent company			(1,109)	(961)
Net cash used in financing activities			(1,109)	(961)
Effect of exchange rate changes on cash			(2)	2
Net increase in cash			74	(7)
Cash at beginning of year			157	141
Cash at end of period	231	134	231	134
Income taxes paid (received)			193	336
Interest paid			24	24
Reportable Legal Entities [Member] | Other Subsidiaries [Member]
Consolidating Statement of Cash Flows
Net income	120	162	288	306
Net adjustments to reconcile net income to net cash provided by operating activities			11	83
Net cash provided by operating activities			299	389
Proceeds from maturities of fixed maturities			914	1,087
Proceeds from sales of investments:
Fixed maturities			759	278
Equity securities			59
Equity securities				75
Real estate investments				20
Other investments			56	57
Purchases of investments:
Fixed maturities			(2,173)	(1,511)
Equity securities			(56)
Equity securities				(41)
Real estate investments			(44)	(26)
Other investments			(43)	(57)
Net sales (purchases) of short-term securities			383	78
Securities transactions in course of settlement			12	64
Other investing activities			(4)	6
Net cash used in investing activities			(137)	30
Issuance of debt			7
Dividends paid to parent company			(170)	(396)
Net cash used in financing activities			(163)	(396)
Effect of exchange rate changes on cash			(2)	5
Net increase in cash			(3)	28
Cash at beginning of year			187	164
Cash at end of period	184	192	184	192
Income taxes paid (received)			171	119
Reportable Legal Entities [Member] | TRV [Member]
Consolidating Statement of Cash Flows
Net income	524	595	1,193	1,212
Net adjustments to reconcile net income to net cash provided by operating activities			146	223
Net cash provided by operating activities			1,339	1,435
Proceeds from maturities of fixed maturities			13	2
Proceeds from sales of investments:
Fixed maturities			1
Equity securities				121
Purchases of investments:
Fixed maturities			(17)	(2)
Equity securities			(2)
Equity securities				(122)
Net sales (purchases) of short-term securities			(167)	(885)
Securities transactions in course of settlement			(1)	1
Net cash used in investing activities			(173)	(885)
Treasury stock acquired — share repurchase authorization			(700)	(700)
Treasury stock acquired — net employee share-based compensation			(51)	(61)
Dividends paid to shareholders			(404)	(389)
Payment of debt			(600)	(207)
Issuance of debt			491	689
Issuance of common stock — employee share options			98	118
Net cash used in financing activities			(1,166)	(550)
Cash at beginning of year				2
Cash at end of period		2		2
Income taxes paid (received)			(126)	(132)
Interest paid			151	154
Eliminations [Member]
Consolidating Statement of Cash Flows
Net income	$ (578)	$ (600)	(1,297)	(1,252)
Net adjustments to reconcile net income to net cash provided by operating activities			25	(105)
Net cash provided by operating activities			(1,272)	(1,357)
Purchases of investments:
Issuance of debt			(7)
Dividends paid to parent company			1,279	1,357
Net cash used in financing activities			$ 1,272	$ 1,357