Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Earnings Per Share
Net income attributable to common shareholders	$ 15,766	$ 24,444
Average common shares outstanding - basic	110,257	109,832
Net effect of dilutive securities:
Contingently issuable performance shares and restricted stock units	631	1,003
Average common shares outstanding - diluted	110,888	110,835
Earnings per average common share attributable to common shareholders -basic (in dollars per share)	$ 0.14	$ 0.22
Earnings per average common share attributable to common shareholders -diluted (in dollars per share)	$ 0.14	$ 0.22