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, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
INCOME
Net income	$ 46,686	$ 70,348	$ 76,718	$ 99,363
Deduct: non-participating stock dividends (4.5% preferred stock)	0	15	0	26
Deduct: non-participating stock redemption costs (4.5% preferred stock)	0	112	0	112
Basic net income applicable to common stock	46,686	70,221	76,718	99,225
Diluted net income applicable to common stock	$ 46,686	$ 70,221	$ 76,718	$ 99,225
SHARES
Average number of basic common shares outstanding (in shares)	60,421,028	60,655,538	60,387,388	60,613,371
Effect of dilutive securities
Add: stock option grants (in shares)	1,504	21,634	4,455	21,067
Add: restricted stock (LTICP) (in shares)	238,170	346,267	233,534	163,107
Average number of diluted common shares outstanding (in shares)	60,660,702	61,023,439	60,625,377	60,797,545
PER SHARE AMOUNT
Net income applicable to common stock (in dollars per share)	$ 0.77	$ 1.16	$ 1.27	$ 1.64
Diluted net income per average common share (in dollars per share)	$ 0.77	$ 1.15	$ 1.27	$ 1.63
Cumulative Preferred Stock, 100 Par Value [Member]
Dividends Payable [Line Items]
Preferred stock, dividend rate			4.50%