Fair value measurements (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Carrying values and the fair values of Senior Notes
Carrying Amount	$ 8,076.1	$ 2,493.6
5.250% senior notes due 2012, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Carrying Amount	999.9	999.6
6.250% senior notes due 2014, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Carrying Amount	997.8	996.9
3.125% senior notes due 2016, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Carrying Amount	1,494.6	0
7.250% senior notes due 2019, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Carrying Amount	497.3	497.1
3.500% senior notes due 2016, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Carrying Amount	1,249.7	0
4.750% senior notes due 2021, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Carrying Amount	1,239.4	0
2.750% senior notes due 2014, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Carrying Amount	899.0	0
6.125% senior notes due 2041, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Carrying Amount	698.4	0
Fair Value, Inputs, Level 1 [Member]
Carrying values and the fair values of Senior Notes
Fair Value	8,413.5	2,758.3
Fair Value, Inputs, Level 1 [Member] | 5.250% senior notes due 2012, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Fair Value	1,017.5	1,056.0
Fair Value, Inputs, Level 1 [Member] | 6.250% senior notes due 2014, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Fair Value	1,085.0	1,116.0
Fair Value, Inputs, Level 1 [Member] | 3.125% senior notes due 2016, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Fair Value	1,493.7	0
Fair Value, Inputs, Level 1 [Member] | 7.250% senior notes due 2019, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Fair Value	593.1	586.3
Fair Value, Inputs, Level 1 [Member] | 3.500% senior notes due 2016, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Fair Value	1,265.3	0
Fair Value, Inputs, Level 1 [Member] | 4.750% senior notes due 2021, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Fair Value	1,295.8	0
Fair Value, Inputs, Level 1 [Member] | 2.750% senior notes due 2014, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Fair Value	907.8	0
Fair Value, Inputs, Level 1 [Member] | 6.125% senior notes due 2041, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Fair Value	$ 755.3	$ 0