Cash, Cash Equivalents and Available for Sale Securities (Tables)

6 Months Ended

Jun. 30, 2015

Cash, Cash Equivalents and Available for Sale Securities

Summary of available for sale securities, including carrying value and estimated fair values

	As of June 30, 2015
	Cost		Fair Value		Carrying Value		Unrealized Gain
U.S. Treasury securities - maturities < 1 year	$	412,278	$	412,320	$	412,320	$	42
Total	$	412,278	$	412,320	$	412,320	$	42

	As of December 31, 2014
	Cost		Fair Value		Carrying Value		Unrealized Loss
U.S. Treasury securities - maturities < 1 year	$	423,859	$	423,746	$	423,746	$	(113	)
Total	$	423,859	$	423,746	$	423,746	$	(113	)

Schedule of changes in accumulated other comprehensive loss associated with the unrealized gain on available-for-sale securities

	Three months ended June 30,					Six months ended June 30,
	2015			2014		2015			2014
Beginning balance	$	(17	)	$	18	$	(65	)	$	—
Current period changes in fair value, net of tax	41			7		89			25
Ending balance	$	24		$	25	$	24		$	25