Summary of Significant Accounting Policies, Earnings Per Average Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Income
Basic net income applicable to common stock	$ 66,407	$ 63,818	$ 135,572	$ 140,536
Diluted net income applicable to common stock	$ 66,407	$ 63,818	$ 135,572	$ 140,536
Shares
Average number of basic common shares outstanding (in shares)	60,450,384	60,346,476	60,428,944	60,375,538
Effect of dilutive securities
Add: stock option grants (in shares)	0	1,536	0	4,518
Add: restricted stock (LTICP) (in shares)	298,263	251,191	265,688	246,415
Average number of diluted common shares outstanding (in shares)	60,748,647	60,599,203	60,694,632	60,626,471
Per Share Amount
Net income applicable to common stock (in dollars per share)	$ 1.10	$ 1.06	$ 2.25	$ 2.33
Diluted net income per average common share (in dollars per share)	$ 1.09	$ 1.05	$ 2.23	$ 2.32
Stock Options [Member]
Per Share Amount
Amount of stock options excluded from computation of diluted earnings per share (in shares)				0