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, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Income
Basic net income applicable to common stock	$ 36,633	$ 42,032	$ 62,557	$ 69,166
Diluted net income applicable to common stock	$ 36,633	$ 42,032	$ 62,557	$ 69,166
Shares
Average number of basic common shares outstanding (in shares)	60,359,949	60,445,617	60,424,591	60,419,588
Effect of dilutive securities
Add: restricted stock (LITCP) (in shares)	266,186	267,757	253,435	250,524
Average number of diluted common shares outstanding (in shares)	60,626,135	60,713,374	60,678,026	60,670,112
Per Share Amount
Net income applicable to common stock (in dollars per share)	$ 0.61	$ 0.70	$ 1.04	$ 1.15
Diluted net income per average common share (in dollars per share)	$ 0.60	$ 0.69	$ 1.03	$ 1.14
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross			0	0