Debt - Additional Information (Details) - USD ($)	1 Months Ended	6 Months Ended
	Mar. 31, 2018	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Jul. 31, 2015
Line of Credit Facility [Line Items]
Long-term debt, gross		$ 2,224,704,000	$ 1,967,627,000
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 440,000,000
Debt term		5 years
Remaining borrowing capacity		$ 200,100,000
Letters of credit issued		38,900,000	47,500,000
Convertible debt [Member]
Line of Credit Facility [Line Items]
Principal of convertible senior notes		$ 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		$ 53,837,000	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]
Unused capacity, commitment fee percentage	0.30%
Contractual Interest Rate		1.10%
Debt Service Reserve Account LC [Member] | Tsugaru Credit Facility [Member]
Line of Credit Facility [Line Items]
Principal of convertible senior notes		$ 19,700,000
O&M Reserve Account LC [Member] | Tsugaru Credit Facility [Member]
Line of Credit Facility [Line Items]
Principal of convertible senior notes		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
Commitment fee percentage	0.50%
Contractual Interest Rate		3.09%
Long-term debt, gross		$ 58,320,000	$ 0
Tsugaru Holdings Loan [Member] | Tokyo Interbank Offered Rate (TIBOR) [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	3.00%