Consolidating Financial Statements (Details) - Consolidating Statement of Cash Flows (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Consolidating Statement of Cash Flows
Net income	$ 669	$ 617
Net adjustments to reconcile net income to net cash provided by operating activities	(115)	231
Net cash provided by operating activities	554	848
Proceeds from maturities of fixed maturities	1,950	2,218
Proceeds from sales of investments:
Fixed maturities	1,085	188
Equity securities	26
Equity securities		21
Real estate investments		11
Other investments	114	122
Purchases of investments:
Fixed maturities	(3,920)	(3,056)
Equity securities	(20)
Equity securities		(22)
Real estate investments	(33)	(16)
Other investments	(142)	(124)
Net sales (purchases) of short-term securities	410	49
Securities transactions in course of settlement	202	157
Other investing activities	(53)	(63)
Net cash used in investing activities	(381)	(515)
Treasury stock acquired — share repurchase authorization	(350)	(225)
Treasury stock acquired — net employee share-based compensation	(51)	(61)
Dividends paid to shareholders	(197)	(190)
Payment of debt	(100)
Issuance of debt	491
Issuance of common stock — employee share options	85	83
Net cash used in financing activities	(122)	(393)
Effect of exchange rate changes on cash	2	2
Net increase (decrease) in cash	53	(58)
Cash at beginning of year	344	307
Cash at end of period	397	249
Income taxes paid	56	2
Interest paid	39	43
Reportable Legal Entities [Member] | TPC [Member]
Consolidating Statement of Cash Flows
Net income	551	508
Net adjustments to reconcile net income to net cash provided by operating activities	(126)	160
Net cash provided by operating activities	425	668
Proceeds from maturities of fixed maturities	1,453	1,638
Proceeds from sales of investments:
Fixed maturities	729	88
Equity securities	8
Equity securities		2
Other investments	76	96
Purchases of investments:
Fixed maturities	(2,836)	(2,191)
Equity securities	(1)
Equity securities		(1)
Other investments	(115)	(96)
Net sales (purchases) of short-term securities	729	245
Securities transactions in course of settlement	147	102
Other investing activities	(52)	(70)
Net cash used in investing activities	138	(187)
Dividends paid to parent company	(544)	(532)
Net cash used in financing activities	(544)	(532)
Effect of exchange rate changes on cash	(1)	1
Net increase (decrease) in cash	18	(50)
Cash at beginning of year	157	141
Cash at end of period	175	91
Income taxes paid	13	1
Interest paid	16	16
Reportable Legal Entities [Member] | Other Subsidiaries [Member]
Consolidating Statement of Cash Flows
Net income	168	144
Net adjustments to reconcile net income to net cash provided by operating activities	(57)	9
Net cash provided by operating activities	111	153
Proceeds from maturities of fixed maturities	493	580
Proceeds from sales of investments:
Fixed maturities	356	100
Equity securities	18
Equity securities		19
Real estate investments		11
Other investments	38	26
Purchases of investments:
Fixed maturities	(1,078)	(864)
Equity securities	(18)
Equity securities		(20)
Real estate investments	(33)	(16)
Other investments	(27)	(28)
Net sales (purchases) of short-term securities	201	233
Securities transactions in course of settlement	56	53
Other investing activities	(1)	7
Net cash used in investing activities	5	101
Dividends paid to parent company	(85)	(263)
Net cash used in financing activities	(85)	(263)
Effect of exchange rate changes on cash	3	1
Net increase (decrease) in cash	34	(8)
Cash at beginning of year	187	164
Cash at end of period	221	156
Income taxes paid	43	1
Reportable Legal Entities [Member] | TRV [Member]
Consolidating Statement of Cash Flows
Net income	669	617
Net adjustments to reconcile net income to net cash provided by operating activities	(22)	205
Net cash provided by operating activities	647	822
Proceeds from maturities of fixed maturities	4
Purchases of investments:
Fixed maturities	(6)	(1)
Equity securities	(1)
Equity securities		(1)
Net sales (purchases) of short-term securities	(520)	(429)
Securities transactions in course of settlement	(1)	2
Net cash used in investing activities	(524)	(429)
Treasury stock acquired — share repurchase authorization	(350)	(225)
Treasury stock acquired — net employee share-based compensation	(51)	(61)
Dividends paid to shareholders	(197)	(190)
Payment of debt	(100)
Issuance of debt	491
Issuance of common stock — employee share options	85	83
Net cash used in financing activities	(122)	(393)
Net increase (decrease) in cash	1
Cash at beginning of year		2
Cash at end of period	1	2
Interest paid	23	27
Eliminations [Member]
Consolidating Statement of Cash Flows
Net income	(719)	(652)
Net adjustments to reconcile net income to net cash provided by operating activities	90	(143)
Net cash provided by operating activities	(629)	(795)
Purchases of investments:
Dividends paid to parent company	629	795
Net cash used in financing activities	$ 629	$ 795