Earnings Per Share (details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Earnings Per Share [Abstract]
Net income	$ 37,884	$ 37,639
Average common shares outstanding - basic	27,094,000	26,929,000
Additional shares for stock based compensation plans	32,000	44,000
Average common shares outstanding - diluted	27,126,000	26,973,000
Earnings per share of common stock - basic	$ 1.40	$ 1.40
Earnings per share of common stock - diluted	$ 1.40	$ 1.40
Earnings Per Share, Diluted, Other Disclosures [Abstract]
Antidilutive Shares Excluded from Net Income Per Diluted Common Share Calculation	44,000	32,000