Debt (details) - Line of Credit (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Line of credit [Member]
Line of credit
Credit agreement, term (in years)	5 years
Credit agreement, maximum borrowing capacity	1,000,000,000
Credit agreement, covenant terms	Pursuant to the credit agreement covenants, the Company must maintain a minimum consolidated net worth, defined as shareholders' equity determined in accordance with GAAP plus (a) trust preferred securities (not to exceed 15% of total capital) and (b) mandatorily convertible securities (combined with trust preferred securities, not to exceed 25% of total capital) less goodwill and other intangible assets, of $13.73 billion. In addition, the credit agreement contains other customary restrictive covenants as well as certain customary events of default, including with respect to a change in control, which is defined to include the acquisition of 35% or more of the Company's voting stock and certain changes in the composition of the Company's board of directors.
Maximum percentage of trust preferred securities relative to total capital in determining consolidated net worth	15.00%
Maximum percentage of trust preferred securities and mandatorily convertible securities relative to total capital in determining consolidated net worth	25.00%
Percentage of Company's voting stock acquired by outside entity that would be considered a change in control	35.00%
Credit agreement, compliance	At December 31, 2014, the Company was in compliance with these covenants.
Line of credit [Member] | Minimum [Member]
Line of credit
Credit agreement, threshold of consolidated net worth	13,730,000,000
Line of credit [Member] | LIBOR [Member]
Line of credit
Credit agreement, cost of borrowing, basis points above LIBOR	1.125%
Line of credit [Member] | LIBOR [Member] | Minimum [Member]
Line of credit
Credit agreement, cost of borrowing, basis points above LIBOR	0.875%
Line of credit [Member] | LIBOR [Member] | Maxiumum [Member]
Line of credit
Credit agreement, cost of borrowing, basis points above LIBOR	1.50%
Commercial Paper [Member]
Line of credit
Credit agreement, maximum borrowing capacity	800,000,000
Commercial Paper [Member] | Minimum [Member]
Line of credit
Interest rate on commercial paper	0.08%	0.08%
Commercial Paper [Member] | Maxiumum [Member]
Line of credit
Interest rate on commercial paper	0.15%	0.13%