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, 2012	Mar. 31, 2011
INCOME
Net income	$ 30,031	$ 29,016
Deduct: non-participating stock dividends (4.5% preferred stock)	0	12
Net income applicable to common stock	30,031	29,004
Diluted net income applicable to common stock	$ 30,031	$ 29,004
SHARES
Average number of basic common shares outstanding (in shares)	60,372,567	60,576,004
Effect of dilutive securities
Add: stock option grants (in shares)	13,097	22,701
Add: restricted stock (LTICP) (in shares)	213,311	306,162
Average number of diluted common shares outstanding (in shares)	60,598,975	60,904,867
PER SHARE AMOUNT
Net income applicable to common stock (in dollars per share)	$ 0.50	$ 0.48
Diluted net income per average common share (in dollars per share)	$ 0.50	$ 0.48
Cumulative Preferred Stock, 100 Par Value [Member]
Dividends Payable [Line Items]
Preferred stock, dividend rate	450.00%