Fair Value Measurements - Changes in balances of Level 3 financial assets carried at fair value (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	$ 2,290	$ 2,290	$ 1,762	$ 1,636
Total realized/unrealized gains (losses) included in Net earnings		(21)	(39)	(35)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)		(39)	17	21
Purchases and issuances		1,828	1,079	347
Sales and settlements		(482)	(437)	(217)
Transfer into Level 3		270	289	139
Transfer out of Level 3		(325)	(381)	(129)
Financial assets, Ending Balance		3,521	2,290	1,762
Fixed maturities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	2,102	2,102	1,559	1,470
Total realized/unrealized gains (losses) included in Net earnings		(22)	(7)	(14)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)		(39)	7	(14)
Purchases and issuances		1,667	1,032	291
Sales and settlements		(476)	(421)	(189)
Transfer into Level 3		240	289	124
Transfer out of Level 3		(308)	(357)	(109)
Financial assets, Ending Balance		3,164	2,102	1,559
U.S. government agency
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	8	8	8	15
Total realized/unrealized gains (losses) included in Net earnings		0	0	(8)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)		0	0	1
Purchases and issuances		0	0	0
Sales and settlements		0	0	0
Transfer into Level 3		1	0	0
Transfer out of Level 3		0	0	0
Financial assets, Ending Balance		9	8	8
State and municipal
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	148	148	140	89
Total realized/unrealized gains (losses) included in Net earnings		0	0	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)		(2)	2	(4)
Purchases and issuances		0	0	57
Sales and settlements		(3)	(2)	(2)
Transfer into Level 3		0	10	0
Transfer out of Level 3		(84)	(2)	0
Financial assets, Ending Balance		59	148	140
Residential MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	122	122	190	224
Total realized/unrealized gains (losses) included in Net earnings		(9)	(4)	(4)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)		(4)	2	(2)
Purchases and issuances		0	1	8
Sales and settlements		(21)	(40)	(28)
Transfer into Level 3		130	44	34
Transfer out of Level 3		(21)	(71)	(42)
Financial assets, Ending Balance		197	122	190
Commercial MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	36	36	25	39
Total realized/unrealized gains (losses) included in Net earnings		0	2	(1)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)		0	0	0
Purchases and issuances		20	15	0
Sales and settlements		0	(10)	(7)
Transfer into Level 3		0	4	0
Transfer out of Level 3		0	0	(6)
Financial assets, Ending Balance		56	36	25
Asset-backed securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	744	744	484	470
Total realized/unrealized gains (losses) included in Net earnings		(3)	0	(1)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)		(15)	1	1
Purchases and issuances		426	410	50
Sales and settlements		(248)	(132)	(52)
Transfer into Level 3		82	202	60
Transfer out of Level 3		(139)	(221)	(44)
Financial assets, Ending Balance		847	744	484
Corporate and other
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	1,044	1,044	712	633
Total realized/unrealized gains (losses) included in Net earnings		(10)	(5)	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)		(18)	2	(10)
Purchases and issuances		1,221	606	176
Sales and settlements		(204)	(237)	(100)
Transfer into Level 3		27	29	30
Transfer out of Level 3		(64)	(63)	(17)
Financial assets, Ending Balance		1,996	1,044	712
Equity securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	165	165	174	140
Total realized/unrealized gains (losses) included in Net earnings		9	(21)	(12)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)		0	10	35
Purchases and issuances		155	38	44
Sales and settlements		(6)	(16)	(28)
Transfer into Level 3	29	30	0	15
Transfer out of Level 3		(17)	(20)	(20)
Financial assets, Ending Balance		336	165	174
Assets of MIE
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	$ 23	23	29	26
Total realized/unrealized gains (losses) included in Net earnings		(8)	(11)	(9)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)		0	0	0
Purchases and issuances		6	9	12
Sales and settlements		0	0	0
Transfer into Level 3		0	0	0
Transfer out of Level 3		0	(4)	0
Financial assets, Ending Balance		$ 21	$ 23	$ 29