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, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
INCOME
Basic net income applicable to common stock	$ 21,347	$ 70,835	$ 36,633	$ 25,924	$ 25,112	$ 66,407	$ 42,032	$ 27,133	$ 154,739	$ 160,685	$ 163,648
Diluted net income applicable to common stock									$ 154,739	$ 160,685	$ 163,648
SHARES
Average number of basic common shares outstanding (in shares)									60,406,001	60,434,510	60,370,588
Effect of dilutive securities
Average number of diluted common shares outstanding (in shares)									60,601,458	60,720,090	60,628,129
PER SHARE AMOUNT
Net income applicable to common stock (in dollars per share)	$ 0.35	$ 1.17	$ 0.61	$ 0.43	$ 0.42	$ 1.10	$ 0.70	$ 0.45	$ 2.56	$ 2.66	$ 2.71
Diluted net income per average common share (in dollars per share)	$ 0.35	$ 1.17	$ 0.60	$ 0.43	$ 0.41	$ 1.09	$ 0.69	$ 0.45	$ 2.55	$ 2.65	$ 2.70
Stock option grants excluded from computation of diluted earnings per share											0
Stock option grants during period (in shares)									0	0	0
Employee Stock Option [Member]
Effect of dilutive securities
Dilutive securities (in shares)									0	0	4,154
LTICP [Member]
Effect of dilutive securities
Dilutive securities (in shares)									195,457	285,580	253,387