Subsequent Event-Stock Split (Details) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 31, 2013	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Earnings per share
Basic - as reported (pre-stock split) (in dollars per share)		$ 0.42	$ 0.17	$ 0.63	$ 0.26
Diluted - as reported (pre-stock split) (in dollars per share)		$ 0.42	$ 0.17	$ 0.62	$ 0.26
Weighted-average shares outstanding
Basic - as reported (pre-stock split) (in shares)		210,349,000	209,512,000	210,250,000	209,320,000
Diluted - as reported (pre-stock split) (in shares)		211,745,000	211,158,000	211,492,000	210,974,000
Subsequent event
Subsequent event-stock split
Stock split ratio declared by the Board of Directors	2
Reduction in additional paid-in capital due to pro forma effect				$ 21.1
Increase in common shares outstanding due to pro forma effect				21,100,000
Earnings per share
Basic - as reported (pre-stock split) (in dollars per share)		$ 0.42	$ 0.17	$ 0.63	$ 0.26
Basic - pro forma (post-stock split) (in dollars per share)		$ 0.21	$ 0.09	$ 0.32	$ 0.13
Diluted - as reported (pre-stock split) (in dollars per share)		$ 0.42	$ 0.17	$ 0.62	$ 0.26
Diluted - pro forma (post-stock split) (in dollars per share)		$ 0.21	$ 0.09	$ 0.31	$ 0.13
Weighted-average shares outstanding
Basic - as reported (pre-stock split) (in shares)		210,349,000	209,512,000	210,250,000	209,320,000
Basic - pro forma (post-stock split) (in shares)		420,698,000	419,024,000	420,500,000	418,640,000
Diluted - as reported (pre-stock split) (in shares)		211,745,000	211,158,000	211,492,000	210,974,000
Diluted - pro forma (post-stock split) (in shares)		423,490,000	422,316,000	422,984,000	421,948,000