Financial Instruments - Cash and Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2019		Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018		Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents	$ 202,823	[1]	$ 226,499	$ 308,009	$ 320,414	$ 220,728	$ 220,728	[1]	$ 395,516	$ 91,596	$ 88,227
Restricted cash	174,565						59,757
Cash, cash equivalents and restricted cash	377,388		$ 357,875	$ 371,066	$ 369,934		280,485		$ 444,226	$ 149,872	$ 143,592	$ 180,612	$ 217,915
Cash
Debt Securities, Available-for-sale [Line Items]
Cash, cash equivalents and restricted cash	100,773						136,642
Money market funds
Debt Securities, Available-for-sale [Line Items]
Cash, cash equivalents and restricted cash	$ 276,615						$ 143,843

[1] We have variable interest entities which represent a portion of the consolidated balances are recorded within these financial statement line items in the Consolidated Balance Sheets (see Note 13, Power Purchase Agreement Programs).