Available-for-Sale Securities (Tables)

6 Months Ended

Jun. 30, 2017

Schedule of Available-for-sale Securities [Line Items]

Schedule of Available-for-sale Securities Reconciliation [Table Text Block]

Available-for-Sale Securities by Major Security Types (in thousands)

	June 30, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Certificates of deposit	$	350,000	$	10	$	(6	)	$	350,004
Total	$	350,000	$	10	$	(6	)	$	350,004
	December 31, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Certificates of deposit	$	1,300,000	$	38	$	(15	)	$	1,300,023
Total	$	1,300,000	$	38	$	(15	)	$	1,300,023

Available-for-sale Securities [Member]

Schedule of Available-for-sale Securities [Line Items]

Investments Classified by Contractual Maturity Date [Table Text Block]

Available-for-Sale Securities by Contractual Maturity (in thousands)

	June 30, 2017				December 31, 2016
Year of Maturity	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Due in one year or less	$	350,000	$	350,004	$	1,300,000	$	1,300,023

Schedule of Interest Rate Payment Terms For Investments [Table Text Block]

Interest Rate Payment Terms of Available-for-Sale Securities (in thousands)

	June 30, 2017		December 31, 2016
Amortized cost of available-for-sale securities:
Fixed-rate	$	350,000	$	1,300,000