Summary of Significant Accounting Policies, Earnings Per Average Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Income
Basic net income applicable to common stock	$ 42,032	$ 46,686	$ 69,166	$ 76,718
Diluted net income applicable to common stock	$ 42,032	$ 46,686	$ 69,166	$ 76,718
Shares
Average number of basic common shares outstanding (in shares)	60,445,617	60,421,028	60,419,588	60,387,388
Effect of dilutive securities
Add: stock option grants (in shares)	0	1,504	0	4,455
Add: restricted stock (LTICP) (in shares)	267,757	238,170	250,524	233,534
Average number of diluted common shares outstanding (in shares)	60,713,374	60,660,702	60,670,112	60,625,377
Per Share Amount
Net income applicable to common stock (in dollars per share)	$ 0.70	$ 0.77	$ 1.15	$ 1.27
Diluted net income per average common share (in dollars per share)	$ 0.69	$ 0.77	$ 1.14	$ 1.27
Stock Options [Member]
Per Share Amount
Amount of stock options excluded from computation of diluted earnings per share (in shares)				0