Exit and Termination Costs - Exit and Termination Costs Included in Accrued Liabilities in Consolidated Balance Sheet - (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Exit costs included in accrued liabilities in consolidated balance sheet
Accrued severance costs	$ 4,191	$ 819	$ 5,638	$ 3,000
Balance at September 30, 2020	496,408		496,408
Exit Costs
Exit costs included in accrued liabilities in consolidated balance sheet
Balance at December 31, 2019			0
Accrued severance costs			0
Accrued contract obligations			490,115
Accretion of discount			7,252
Payments			(959)
Balance at September 30, 2020	496,408		496,408
Termination Costs
Exit costs included in accrued liabilities in consolidated balance sheet
Balance at December 31, 2019			4,692
Accrued severance costs			5,638
Accrued contract obligations			0
Accretion of discount			0
Payments			(6,170)
Balance at September 30, 2020	$ 4,160		$ 4,160