Summary of Significant Accounting Policies, Earnings Per Average Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accounting Policies [Abstract]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount									0	0	0
INCOME
Basic net income applicable to common stock	$ 21,850	$ 54,663	$ 30,234	$ 26,922	$ 21,347	$ 70,835	$ 36,633	$ 25,924	$ 133,669	$ 154,739	$ 160,685
Diluted net income applicable to common stock									$ 133,669	$ 154,739	$ 160,685
SHARES
Basic average number of common shares outstanding (in shares)									60,476,066	60,406,001	60,434,510
Effect of dilutive securities
Add: restricted (LTICP) (in shares)									213,203	195,457	285,580
Average number of diluted common shares outstanding (in shares)									60,689,269	60,601,458	60,720,090
PER SHARE AMOUNT
Basic net income applicable to common stock (in dollars per share)	$ 0.36	$ 0.90	$ 0.50	$ 0.45	$ 0.35	$ 1.17	$ 0.61	$ 0.43	$ 2.21	$ 2.56	$ 2.66
Diluted net income applicable to common share (in dollars per share)	$ 0.36	$ 0.90	$ 0.50	$ 0.44	$ 0.35	$ 1.17	$ 0.60	$ 0.43	$ 2.20	$ 2.55	$ 2.65