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, 2015	Mar. 31, 2014
Income
Basic net income applicable to common stock	$ 26,922	$ 25,924
Diluted net income applicable to common stock	$ 26,922	$ 25,924
Shares
Average number of basic common shares outstanding (in shares)	60,463,693	60,472,969
Effect of dilutive securities
Add: restricted stock (LITCP) (in shares)	236,392	240,618
Average number of diluted common shares outstanding (in shares)	60,700,085	60,713,587
Per Share Amount
Net income applicable to common stock (in dollars per share)	$ 0.45	$ 0.43
Diluted net income per average common share (in dollars per share)	$ 0.44	$ 0.43