Fair value measurements (Details) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Carrying values and the fair values of Senior Notes
Carrying Amount	$ 3,989.0	$ 2,493.6
Fair Value, Inputs, Level 1 [Member]
Carrying values and the fair values of Senior Notes
Fair Value	4,222.4	2,758.3
Fair Value, Inputs, Level 1 [Member] | 5.25% senior notes due 2012, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Fair Value	1,027.5	1,056.0
Fair Value, Inputs, Level 1 [Member] | 6.25% senior notes due 2014, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Fair Value	1,095.2	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	1,492.1	0
Fair Value, Inputs, Level 1 [Member] | 7.25% senior notes due 2019, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Fair Value	607.6	586.3
5.25% senior notes due 2012, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Carrying Amount	999.8	999.6
6.25% senior notes due 2014, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Carrying Amount	997.6	996.9
3.125% senior notes due 2016, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Carrying Amount	1,494.4	0
7.25% senior notes due 2019, net of unamortized discount [Member]
Carrying values and the fair values of Senior Notes
Carrying Amount	$ 497.2	$ 497.1