Consolidating Financial Statements (Details) - Consolidating Statement of Cash Flows (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Consolidating Statement of Cash Flows
Net income	$ 709	$ 293	$ 1,902	$ 1,505
Net adjustments to reconcile net income to net cash provided by operating activities			1,530	1,980
Net cash provided by operating activities			3,432	3,485
Proceeds from maturities of fixed maturities			5,655	6,581
Proceeds from sales of investments:
Fixed maturities			3,185	860
Equity securities			127
Equity securities				340
Real estate investments			8	23
Other investments			270	341
Purchases of investments:
Fixed maturities			(10,862)	(8,403)
Equity securities			(86)
Equity securities				(193)
Real estate investments			(57)	(40)
Other investments			(392)	(392)
Net sales (purchases) of short-term securities			456	(990)
Securities transactions in course of settlement			173	122
Acquisition, net of cash acquired			(4)	(439)
Other investing activities			(232)	(186)
Net cash used in investing activities			(1,759)	(2,376)
Treasury stock acquired — share repurchase authorization			(1,100)	(1,028)
Treasury stock acquired — net employee share-based compensation			(51)	(61)
Dividends paid to shareholders			(611)	(589)
Payment of debt			(600)	(207)
Issuance of debt			591	689
Issuance of common stock — employee share options			117	148
Net cash used in financing activities			(1,654)	(1,048)
Effect of exchange rate changes on cash			(4)	11
Net increase in cash			15	72
Cash at beginning of year			344	307
Cash at end of period	359	379	359	379
Income taxes paid (received)			244	467
Interest paid			225	217
Reportable Legal Entities [Member] | TPC [Member]
Consolidating Statement of Cash Flows
Net income	559	295	1,568	1,241
Net adjustments to reconcile net income to net cash provided by operating activities			1,156	1,140
Net cash provided by operating activities			2,724	2,381
Proceeds from maturities of fixed maturities			4,197	4,961
Proceeds from sales of investments:
Fixed maturities			2,219	463
Equity securities			39
Equity securities				18
Other investments			198	260
Purchases of investments:
Fixed maturities			(7,625)	(5,906)
Equity securities			(3)
Equity securities				(5)
Real estate investments			(1)
Other investments			(329)	(305)
Net sales (purchases) of short-term securities			364	(488)
Securities transactions in course of settlement			113	18
Other investing activities			(225)	(191)
Net cash used in investing activities			(1,053)	(1,175)
Dividends paid to parent company			(1,653)	(1,185)
Net cash used in financing activities			(1,653)	(1,185)
Effect of exchange rate changes on cash			(1)	3
Net increase in cash			17	24
Cash at beginning of year			157	141
Cash at end of period	174	165	174	165
Income taxes paid (received)			198	493
Interest paid			40	40
Reportable Legal Entities [Member] | Other Subsidiaries [Member]
Consolidating Statement of Cash Flows
Net income	172	34	460	340
Net adjustments to reconcile net income to net cash provided by operating activities			373	604
Net cash provided by operating activities			833	944
Proceeds from maturities of fixed maturities			1,439	1,618
Proceeds from sales of investments:
Fixed maturities			965	397
Equity securities			82
Equity securities				202
Real estate investments			8	23
Other investments			72	94
Purchases of investments:
Fixed maturities			(3,215)	(2,493)
Equity securities			(74)
Equity securities				(65)
Real estate investments			(56)	(40)
Other investments			(63)	(87)
Net sales (purchases) of short-term securities			209	(230)
Securities transactions in course of settlement			60	103
Acquisition, net of cash acquired			(4)	25
Other investing activities			(7)	5
Net cash used in investing activities			(584)	(448)
Issuance of debt			7
Dividends paid to parent company			(255)	(454)
Net cash used in financing activities			(248)	(454)
Effect of exchange rate changes on cash			(3)	8
Net increase in cash			(2)	50
Cash at beginning of year			187	164
Cash at end of period	185	214	185	214
Income taxes paid (received)			172	174
Reportable Legal Entities [Member] | TRV [Member]
Consolidating Statement of Cash Flows
Net income	709	302	1,902	1,514
Net adjustments to reconcile net income to net cash provided by operating activities			(126)	285
Net cash provided by operating activities			1,776	1,799
Proceeds from maturities of fixed maturities			19	2
Proceeds from sales of investments:
Fixed maturities			1
Equity securities			6
Equity securities				120
Purchases of investments:
Fixed maturities			(22)	(4)
Equity securities			(9)
Equity securities				(123)
Net sales (purchases) of short-term securities			(117)	(272)
Securities transactions in course of settlement				1
Acquisition, net of cash acquired				(477)
Net cash used in investing activities			(122)	(753)
Treasury stock acquired — share repurchase authorization			(1,100)	(1,028)
Treasury stock acquired — net employee share-based compensation			(51)	(61)
Dividends paid to shareholders			(611)	(589)
Payment of debt			(600)	(207)
Issuance of debt			591	689
Issuance of common stock — employee share options			117	148
Net cash used in financing activities			(1,654)	(1,048)
Net increase in cash				(2)
Cash at beginning of year				2
Income taxes paid (received)			(126)	(200)
Interest paid			185	177
Eliminations [Member]
Consolidating Statement of Cash Flows
Net income	$ (731)	$ (338)	(2,028)	(1,590)
Net adjustments to reconcile net income to net cash provided by operating activities			127	(49)
Net cash provided by operating activities			(1,901)	(1,639)
Proceeds from sales of investments:
Other investments				(13)
Purchases of investments:
Acquisition, net of cash acquired				13
Issuance of debt			(7)
Dividends paid to parent company			1,908	1,639
Net cash used in financing activities			$ 1,901	$ 1,639