Summary of Significant Accounting Policies, Earnings Per Average Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Income
Basic net income applicable to common stock	$ 27,133	$ 30,031
Diluted net income applicable to common stock	$ 27,133	$ 30,031
Shares
Average number of basic common shares outstanding (in shares)	60,399,697	60,372,567
Effect of dilutive securities
Add: stock option grants (in shares)	0	13,097
Add: restricted stock (LTICP) (in shares)	267,704	213,311
Average number of diluted common shares outstanding (in shares)	60,667,401	60,598,975
Per Share Amount
Net income applicable to common stock (in dollars per share)	$ 0.45	$ 0.50
Diluted net income per average common share (in dollars per share)	$ 0.45	$ 0.50
Stock Options [Member]
Per Share Amount
Amount of stock options excluded from computation of diluted earnings per share (in shares)		0