Earnings Per Common Share (Net Income, Weighted Average Common Shares Used in Calculating Basic Earnings Per Common Share and Diluted EPS) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Numerator
Net income	$ 169,349	$ 32,789	$ 389,667	$ 82,833
Deduct dividends paid	(32,519)	(21,616)	(64,966)	(40,450)
Undistributed earnings	$ 136,830	$ 11,173	$ 324,701	$ 42,383
Denominator
Weighted average shares outstanding used in basic earnings per share (in shares)	62,244,778	68,558,472	63,665,856	69,161,001
Incremental common shares from:
Weighted average shares used in diluted earnings per share calculations (in shares)	62,723,292	69,244,399	64,274,009	69,946,364
Earnings per common share - Basic
Distributed earnings, basic (in dollars per share)	$ 0.52	$ 0.30	$ 1.02	$ 0.57
Undistributed earnings, basic (in dollars per share)	2.20	0.18	5.10	0.63
Net income - basic (in dollars per share)	2.72	0.48	6.12	1.20
Earnings per common share - Diluted
Distributed earnings, diluted (in dollars per share)	0.52	0.30	1.01	0.57
Undistributed earnings, diluted (in dollars per share)	2.18	0.17	5.05	0.61
Net income - diluted (in dollars per share)	$ 2.70	$ 0.47	$ 6.06	$ 1.18
PSUs
Incremental common shares from:
Incremental common shares (in shares)	431,575	621,327	561,214	720,763
ESPPs
Incremental common shares from:
Incremental common shares (in shares)	46,939	64,600	46,939	64,600