Fair Value (Tables)

6 Months Ended

Jun. 30, 2020

Fair Value Disclosures [Abstract]

Summary of Fair Value Hierarchy of Assets Carried at Fair Value and Measured on a Recurring Basis

The fair value hierarchy of assets carried at fair value and measured on a recurring basis was as follows: 

	Fair value measurement using:
(dollars in thousands)	Quoted prices in active markets for identical assets (Level 1)		Other observable inputs (Level 2)		Significant unobservable inputs (Level 3)		Total
June 30, 2020
Short-term investments—U.S. Treasury securities	$	778,348	$	—	$	—	$	778,348
December 31, 2019
Short-term investments—U.S. Treasury securities	$	536,493	$	—	$	—	$	536,493
Long-term investments—U.S. Treasury securities	57,283		—		—		57,283
Other non-current assets—equity securities	163,936		—		—		163,936
Total	$	757,712	$	—	$	—	$	757,712

Summary of Debt Securities

Our debt securities consisted of the following:

(dollars in thousands)	Amortized cost		Gross unrealized gains		Gross unrealized losses		Fair value
June 30, 2020
U.S. Treasury securities	$	776,906	$	1,527	$	(85)	$	778,348
Contractual maturities (at date of purchase):
Due in one year or less	$	649,150					$	649,713
Due in one to two years	127,756						128,635
Total	$	776,906					$	778,348
December 31, 2019
U.S. Treasury securities	$	593,565	$	236	$	(25)	$	593,776
Contractual maturities (at date of purchase):
Due in one year or less	$	466,439					$	466,547
Due in one to two years	127,126						127,229
Total	$	593,565					$	593,776