Summary of Significant Accounting Policies, Earnings Per Average Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
INCOME
Net income									$ 163,648	$ 195,848	$ 255,391
Deduct: non-participating stock dividends (4.5% preferred stock)									0	26	46
Deduct: non-participating stock redemption costs (4.5% preferred stock)									0	112	0
Net income applicable to common stock	23,112	63,818	46,686	30,031	30,643	65,842	70,221	29,004	163,648	195,710	255,345
Diluted net income applicable to common stock									$ 163,648	$ 195,710	$ 255,345
SHARES
Average number of basic common shares outstanding (in shares)									60,370,588	60,488,740	60,431,142
Effect of dilutive securities
Add: stock option grants (in shares)									4,154	20,647	32,080
Add: restricted stock (LTICP) (in shares)									253,387	324,177	291,367
Average number of diluted common shares outstanding (in shares)									60,628,129	60,833,564	60,754,589
PER SHARE AMOUNT
Net income applicable to common stock (in dollars per share)	$ 0.38	$ 1.06	$ 0.77	$ 0.50	$ 0.51	$ 1.09	$ 1.16	$ 0.48	$ 2.71	$ 3.24	$ 4.23
Diluted net income per average common share (in dollars per share)	$ 0.38	$ 1.05	$ 0.77	$ 0.50	$ 0.51	$ 1.08	$ 1.15	$ 0.48	$ 2.70	$ 3.22	$ 4.20