Equity-Based Compensation (Narrative) (Details) (USD $)	3 Months Ended		12 Months Ended	3 Months Ended	12 Months Ended				3 Months Ended		12 Months Ended	3 Months Ended			12 Months Ended
	Mar. 31, 2013 Y	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2013 2005 Equity Incentive Plan [Member]	Dec. 31, 2012 Performance Stock Units [Member]	Mar. 31, 2013 Performance Stock Units [Member]	Jan. 31, 2013 Performance Stock Units [Member]	Jan. 31, 2012 Performance Stock Units [Member]	Mar. 31, 2013 Management Stock Purchase Plan [Member]	Mar. 31, 2012 Management Stock Purchase Plan [Member]	Dec. 31, 2012 Management Stock Purchase Plan [Member]	Mar. 31, 2013 Minimum [Member] Performance Stock Units [Member]	Mar. 31, 2013 Maximum [Member] Performance Stock Units [Member]	Mar. 31, 2013 Unvested Restricted Stock Units [Member]	Dec. 31, 2012 Unvested Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum number of shares that may be issued under the Plan				3,000,000
Maximum number of shares that may be issued in connection with grants of incentive stock options and rights				900,000
Performance stock units outstanding						280,000	303,917	295,000
Performance stock unit grant-date fair value							$ 4,123,000	$ 4,152,000
Performance stock unit vesting period												1 year	3 years
Percentage of target performance stock units earned					58.30%
Performance stock units earned					163,200
Weighted average fair value of per unit awarded									$ 13.64		$ 12.30
Number of restricted stock units credited to participant accounts	960,357		777,159						183,178	222,551				93,490	71,992
MSPP company matching portion, vesting age	60
Accrues interest									2.00%
Disbursements of MSPP	$ 531,000	$ 542,000