Summary of Significant Accounting Policies, Earnings Per Average Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income [Abstract]
Basic net income applicable to common stock	$ 54,663	$ 70,835	$ 111,819	$ 133,392
Diluted net income applicable to common stock	$ 54,663	$ 70,835	$ 111,819	$ 133,392
Shares [Abstract]
Basic average number of common shares outstanding (in shares)	60,481,584	60,372,569	60,474,228	60,410,122
Effect of dilutive securities [Abstract]
Add: restricted stock (LITCP) (in shares)	311,807	317,027	285,711	301,421
Average number of diluted common shares outstanding (in shares)	60,793,391	60,689,596	60,759,939	60,711,543
Per Share Amount [Abstract]
Basic net income applicable to common stock (in dollars per share)	$ 0.90	$ 1.17	$ 1.85	$ 2.21
Diluted net income applicable to common stock (in dollars per share)	$ 0.90	$ 1.17	$ 1.84	$ 2.20