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, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
INCOME
Net income	$ 63,818	$ 65,842	$ 140,536	$ 165,205
Deduct: non-participating stock dividends (4.5% preferred stock)			0	26
Deduct: non-participating stock redemption costs (4.5% preferred stock)			0	112
Basic net income applicable to common stock	63,818	65,842	140,536	165,067
Diluted net income applicable to common stock	$ 63,818	$ 65,842	$ 140,536	$ 165,067
SHARES
Average number of basic common shares outstanding (in shares)	60,346,476	60,467,595	60,375,538	60,549,860
Effect of dilutive securities
Add: stock option grants (in shares)	1,536	20,441	4,518	20,965
Add: restricted stock (LTICP) (in shares)	251,191	385,275	246,415	259,426
Average number of diluted common shares outstanding (in shares)	60,599,203	60,873,311	60,626,471	60,830,251
PER SHARE AMOUNT
Net income applicable to common stock (in dollars per share)	$ 1.06	$ 1.09	$ 2.33	$ 2.73
Diluted net income per average common share (in dollars per share)	$ 1.05	$ 1.08	$ 2.32	$ 2.71
Cumulative Preferred Stock, 100 Par Value [Member]
Dividends Payable [Line Items]
Preferred stock, dividend rate			4.50%