Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Earnings Per Share [Abstract]
Net income for basic and diluted earnings per share (in dollars)	$ 137,673	$ 24,747	$ 164,130	$ 100,973
Weighted-average common shares outstanding	222,189	219,502	221,745	219,301
Dilutive effect of stock options, performance share awards and restricted stock awards (in shares)	1,192	1,146	1,310	1,295
Diluted weighted-average shares outstanding	223,381	220,648	223,055	220,596
Basic earnings per share (in dollars per share)	$ 0.62	$ 0.11	$ 0.74	$ 0.46
Diluted earnings per share (in dollars per share)	$ 0.62	$ 0.11	$ 0.74	$ 0.46