Indemnity Shareholders' Equity and Noncontrolling Interest (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Reconciliation of the beginning and ending balances of shareholders' equity and noncontrolling interest
Balance, beginning of period	$ 6,791	$ 6,293	$ 6,293
Net income	290	354	619
Change in other comprehensive (loss) income, net of tax	(22)	72	150
Net purchase of treasury stock	(14)		(69)
Dividends declared:
Balance, end of period	7,017		6,791
Class A
Dividends declared:
Dividends declared	(28)		(200)
Class B
Dividends declared:
Dividends declared	0		(2)
Indemnity
Reconciliation of the beginning and ending balances of shareholders' equity and noncontrolling interest
Balance, beginning of period	642	781	781
Net income	37		160
Change in other comprehensive (loss) income, net of tax	(1)		(28)
Net purchase of treasury stock	(14)		(69)
Dividends declared:
Balance, end of period	636		642
Indemnity | Class A
Dividends declared:
Dividends declared (in dollars per share)	$ 0.5925	$ 0.5525	$ 4.25
Dividends declared	(28)		(200)
Indemnity | Class B
Dividends declared:
Dividends declared (in dollars per share)	$ 88.8750	$ 82.8750	$ 637.5000
Dividends declared	0		(2)
Exchange
Reconciliation of the beginning and ending balances of shareholders' equity and noncontrolling interest
Balance, beginning of period	6,149	5,512	5,512
Net income	253		459
Change in other comprehensive (loss) income, net of tax	(21)		178
Dividends declared:
Balance, end of period	$ 6,381		$ 6,149