Debt - Additional Information (Details) - USD ($)	1 Months Ended		9 Months Ended
	Aug. 31, 2018	Mar. 31, 2018	Sep. 30, 2018	Dec. 31, 2017	Jul. 31, 2015
Line of Credit Facility [Line Items]
Long-term debt, gross			$ 2,418,271,000	$ 1,967,627,000
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity			$ 440,000,000.0
Debt term			5 years
Remaining borrowing capacity			$ 213,200,000
Letters of credit issued			40,400,000	47,500,000
Japan Credit Facility [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 30,800,000.0
Remaining borrowing capacity			7,000,000
Commitment fee percentage	0.30%
Convertible debt [Member]
Line of Credit Facility [Line Items]
Principal			$ 225,000,000	225,000,000	$ 225,000,000
Contractual interest rate			4.00%		4.00%
Tsugaru Construction Loan [Member]
Line of Credit Facility [Line Items]
Contractual interest rate			0.72%
Long-term debt, gross			$ 90,030,000	0
Tsugaru Construction Loan [Member] | Tsugaru Credit Facility [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity			$ 371,400,000.0
Debt term, following conversion		18 years
Tsugaru Construction Loan [Member] | Tokyo Interbank Offered Rate (TIBOR) [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate		0.65%
Tsugaru Letter Of Credit Facilities [Member] | Tsugaru Credit Facility [Member]
Line of Credit Facility [Line Items]
Contractual interest rate			1.10%
Unused capacity, commitment fee percentage		0.30%
Debt Service Reserve Account LC [Member] | Tsugaru Credit Facility [Member]
Line of Credit Facility [Line Items]
Principal			$ 19,700,000
O&M Reserve Account LC [Member] | Tsugaru Credit Facility [Member]
Line of Credit Facility [Line Items]
Principal			8,000,000
Japanese Consumption Tax Facility [Member]
Line of Credit Facility [Line Items]
Commitment fee percentage		0.30%
Japanese Consumption Tax Facility [Member] | Tsugaru Credit Facility [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity			33,800,000.0
Japanese Consumption Tax Facility [Member] | Tokyo Interbank Offered Rate (TIBOR) [Member] | Tsugaru Credit Facility [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate		0.30%
Tsugaru Holdings Loan [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity			$ 70,100,000.0
Contractual interest rate			3.13%
Commitment fee percentage		0.50%
Long-term debt, gross			$ 57,024,000	$ 0
Tsugaru Holdings Loan [Member] | Tokyo Interbank Offered Rate (TIBOR) [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate		3.00%
Minimum [Member] | Tokyo Interbank Offered Rate (TIBOR) [Member] | Japan Credit Facility [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.75%
Maximum [Member] | Tokyo Interbank Offered Rate (TIBOR) [Member] | Japan Credit Facility [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	2.25%