Consolidated Statements of Changes in Common Stockholders' Equity - USD ($) $ in Thousands	Total		Parent [Member]	Common Stock	Treasury Stock	Additional Paid-in Capital	Retained Earnings/(Deficit)	Accumulated Other Comprehensive Income (Loss) [Member]
Balance, at Dec. 31, 2013				$ 191	$ (892,807)	$ 2,347,104	$ (552,226)	$ 37,383
Stock Issued During Period, Value, Conversion of Convertible Securities				0		0
Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures				0		1,141
Issuance of common stock - stock offering				18		247,170
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition						12,176
Equity Impact of Termination of Capped Call Transaction				0		0
Stock Repurchased During Period, Value				0		0
Repurchases of common stock under incentive plans					(154)
Dividends declared						(1,388)
Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt, Subsequent Adjustments						(74,690)
Net income	$ 959,517						959,517
Dividends, Common Stock							(477)
Net foreign currency translation adjustment, net of tax	(226)							(226)
Net unrealized gain on investments, net of tax								14,689
Other Comprehensive Income (Loss), Before Tax, Unrealized Holding Gains (Losses) Arising During the Period on Investments Recorded as Assets Held for Sale, Net of Reclassification Adjustments								(302)
Net actuarial gains (losses)	(59)							(59)
Balance, at Dec. 31, 2014			$ 2,097,060	209	(892,961)	2,531,513	406,814	51,485
Stock Issued During Period, Value, Conversion of Convertible Securities				28		336,358
Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures				1		2,422
Issuance of common stock - stock offering				0		0
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition						15,513
Equity Impact of Termination of Capped Call Transaction				(3)		13,153
Stock Repurchased During Period, Value				(11)		(201,989)
Repurchases of common stock under incentive plans					(215)
Dividends declared						0
Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt, Subsequent Adjustments						(19,648)
Net income	286,924	[1]					286,924
Dividends, Common Stock							(1,996)
Net foreign currency translation adjustment, net of tax	(217)							(217)
Net unrealized gain on investments, net of tax								(66,756)
Other Comprehensive Income (Loss), Before Tax, Unrealized Holding Gains (Losses) Arising During the Period on Investments Recorded as Assets Held for Sale, Net of Reclassification Adjustments								(3,254)
Net actuarial gains (losses)	265							265
Balance, at Dec. 31, 2015	2,496,931		2,496,931	224	(893,176)	2,716,618	691,742	(18,477)
Stock Issued During Period, Value, Conversion of Convertible Securities				17		143,078
Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures				0		2,117
Issuance of common stock - stock offering				0		0
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition						18,257
Equity Impact of Termination of Capped Call Transaction				0		0
Stock Repurchased During Period, Value				(9)		(100,179)
Repurchases of common stock under incentive plans					(156)
Dividends declared						0
Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt, Subsequent Adjustments						0
Net income	308,253						308,253
Dividends, Common Stock							(2,105)
Net foreign currency translation adjustment, net of tax	(474)							(474)
Net unrealized gain on investments, net of tax								6,531
Other Comprehensive Income (Loss), Before Tax, Unrealized Holding Gains (Losses) Arising During the Period on Investments Recorded as Assets Held for Sale, Net of Reclassification Adjustments								0
Net actuarial gains (losses)	25							25
Balance, at Dec. 31, 2016	$ 2,872,286		$ 2,872,286	$ 232	$ (893,332)	$ 2,779,891	$ 997,890	$ (12,395)

[1] For all periods presented, reflects changes to align our segment reporting structure with recent changes in personnel reporting lines and management oversight related to contract underwriting performed on behalf of third parties. Revenue and expenses for this business are now reflected in the Services segment. As a result, for all periods presented, Services revenue and cost of services have increased, with offsetting reductions in other operating expenses.