Acquisition charges (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Business Combination, Separately Recognized Transactions [Line Items]
Acquisition charges	$ 83	$ 86	$ 248	$ 257
National
Business Combination, Separately Recognized Transactions [Line Items]
Acquisition charges	83	86	248	257
National | Amortization of intangible assets
Business Combination, Separately Recognized Transactions [Line Items]
Acquisition charges	80	81	239	244
National | Stock-based compensation
Business Combination, Separately Recognized Transactions [Line Items]
Acquisition charges	3	3	9	8
National | Retention bonuses
Business Combination, Separately Recognized Transactions [Line Items]
Acquisition charges	$ 0	$ 2	$ 0	$ 5