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Investments (Tables)
 12 Months Ended
Dec. 31, 2018
Debt Securities, Available-for-sale [Abstract]  
Summary of Investments The following is a summary of our available-for-sale investment securities, excluding marketable equity securities and those securities classified within cash and cash equivalents on the consolidated balance sheets (in thousands):
 
December 31, 2018
 
Amortized
Cost
 
Gross
Unrealized
Gains
 
Gross
Unrealized
Losses
 
Estimated
Fair Value
Available-for-sale securities:
 
 
 
 
 
 
 
Commercial paper
$
108,061

 
$
—

 
$
—

 
$
108,061

Corporate notes and bonds
1,233,589

 
343

 
(4,218
)
 
1,229,714

Certificates of deposit
73,584

 
1

 
—

 
73,585

U.S. government and agency securities
102,549

 
23

 
(358
)
 
102,214

Total available-for-sale securities
$
1,517,783

 
$
367

 
$
(4,576
)
 
$
1,513,574



 
December 31, 2017
 
Amortized
Cost
 
Gross
Unrealized
Gains
 
Gross
Unrealized
Losses
 
Estimated
Fair Value
Available-for-sale securities:
 
 
 
 
 
 
 
Commercial paper
$
258,348

 
$
1

 
$
(5
)
 
$
258,344

Corporate notes and bonds
1,006,302

 
26

 
(3,084
)
 
1,003,244

Certificates of deposit
33,084

 
—

 
—

 
33,084

U.S. government and agency securities
129,494

 
—

 
(638
)
 
128,856

Total available-for-sale securities
$
1,427,228

 
$
27

 
$
(3,727
)
 
$
1,423,528
Investments Classified by Contractual Maturity Date The fair values of available-for-sale investment securities, by remaining contractual maturity, are as follows (in thousands):
 
December 31, 2018
Due within 1 year
$
931,718

Due in 1 year through 5 years
581,856

Total
$
1,513,574
Fair Values and Gross Unrealized Losses of Available-for-Sale Securities Aggregated by Investment Category The following table shows the fair values and the gross unrealized losses of these securities, classified by the length of time that the securities have been in a continuous unrealized loss position, and aggregated by investment types, excluding those securities classified within cash and cash equivalents on the consolidated balance sheets (in thousands): 
 
December 31, 2018
 
Less than 12 Months
 
12 Months or Greater
 
Total
 
Fair Value
 
Gross
Unrealized
Losses
 
Fair Value
 
Gross
Unrealized
Losses
 
Fair Value
 
Gross
Unrealized
Losses
Corporate notes and bonds
$
714,605

 
$
(2,603
)
 
$
294,956

 
$
(1,615
)
 
$
1,009,561

 
$
(4,218
)
Certificates of deposit
1,000

 
—

 
—

 
—

 
1,000

 
—

U.S. government and agency securities
11,756

 
(5
)
 
61,457

 
(353
)
 
73,213

 
(358
)
Total
$
727,361

 
$
(2,608
)
 
$
356,413

 
$
(1,968
)
 
$
1,083,774

 
$
(4,576
)


 
December 31, 2017
 
Less than 12 Months
 
12 Months or Greater
 
Total
 
Fair Value
 
Gross
Unrealized
Losses
 
Fair Value
 
Gross
Unrealized
Losses
 
Fair Value
 
Gross
Unrealized
Losses
Commercial paper
$
14,809

 
$
(5
)
 
$
—

 
$
—

 
$
14,809

 
$
(5
)
Corporate notes and bonds
819,113

 
(2,703
)
 
141,874

 
(381
)
 
960,987

 
(3,084
)
U.S. government and agency securities
106,301

 
(593
)
 
22,555

 
(45
)
 
128,856

 
(638
)
Total
$
940,223

 
$
(3,301
)
 
$
164,429

 
$
(426
)
 
$
1,104,652

 
$
(3,727
)