Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Numerator:
Net income available to common stockholders for basic and diluted net income per common share	$ 173,015	$ 102,849	$ 344,354	$ 208,541
Denominator:
Weighted average common shares outstanding for basic net income per common share (in shares)	4,938,820	5,443,590	5,002,070	5,506,818
Weighted average impact of dilutive equity instruments (in shares)	49,427	64,011	47,501	63,627
Weighted average shares for diluted net income per common share (in shares)	4,988,247	5,507,601	5,049,571	5,570,445
Net income per common share:
Basic (in dollars per share)	$ 0.04	$ 0.02	$ 0.07	$ 0.04
Diluted (in dollars per share)	$ 0.03	$ 0.02	$ 0.07	$ 0.04