Earnings Per Share (details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended									6 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2012	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings Per Share [Abstract]
Net income	$ 37,639	$ 28,147	$ (10,879)	$ 1,227	$ 40,284	$ 28,766	$ (8,527)	$ 1,985	$ 40,820	$ 41,511	$ 30,632	$ 58,779	$ 63,044	$ 72,013
Average common shares outstanding - basic	26,929,000				26,781,000
Additional shares for stock based compensation plans	44,000				81,000
Average common shares outstanding - diluted	26,973,000				26,862,000
Earnings per share of common stock - basic	$ 1.40	$ 1.05	$ (0.41)	$ 0.05	$ 1.50	$ 1.08	$ (0.32)	$ 0.07	$ 1.53	$ 1.55	$ 1.14	$ 2.19	$ 2.36	$ 2.71
Earnings per share of common stock - diluted	$ 1.40	$ 1.04	$ (0.41)	$ 0.05	$ 1.50	$ 1.07	$ (0.32)	$ 0.07	$ 1.53	$ 1.54	$ 1.14	$ 2.18	$ 2.36	$ 2.70
Earnings Per Share, Diluted, Other Disclosures [Abstract]
Antidilutive Shares Excluded from Net Income Per Diluted Common Share Calculation	32,000				1,000