Acquisition charges (Details) (National, USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Business Combination, Separately Recognized Transactions [Line Items]
Acquisition charges	$ 82	$ 86	$ 165	$ 171
Amortization of intangible assets
Business Combination, Separately Recognized Transactions [Line Items]
Acquisition charges	79	81	159	162
Stock-based compensation
Business Combination, Separately Recognized Transactions [Line Items]
Acquisition charges	3	3	6	6
Retention bonuses
Business Combination, Separately Recognized Transactions [Line Items]
Acquisition charges	$ 0	$ 2	$ 0	$ 3