Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2011	Dec. 31, 2012
Basic EPS Two Class Method:
Income	$ 7,209	$ 3,570	$ 20,939
Income, share amounts	43,167,000	42,674,000
Less: Income allocated to unvested shares	(165)	(74)
Income Allocated to Common Shares	7,044	3,496
Income allocated to common shares, shares	43,167,000	42,674,000
Earnings Per Share, Basic	$ 0.16	$ 0.08
Dilutive Securities:
Plus: Income allocated to unvested shares	165	74
Income Reallocated to Unvested Shareholders	(165)	(74)
Incremental Stock Options Attributable to Share-based Payment Arrangements	34,000	200,000
Incremental Common Shares Attributable to Contingently Issuable Shares	122,000	0
Diluted EPS:
Income Allocated to Common Shares, and Assumed Share Conversions	$ 7,044	$ 3,496
Income Allocated to Common Shares, and Assumed Share Conversions, shares	43,323,000	42,874,000
Earnings Per Share, Diluted	$ 0.16	$ 0.08
Earnings Per Share (Textual)
Stock options to purchase shares, shares	1,562,899	1,700,000		1,585,594
Stock options to purchase shares, price per share	$ 32.96	$ 32.56
Stock Options [Member]
Earnings Per Share (Textual)
Antidilutive shares excluded from EPS, shares	1,400,000	900,000
Restricted Stock Units (RSUs) [Member]
Earnings Per Share (Textual)
Antidilutive shares excluded from EPS, shares	300,000