Financial Instruments - Narrative (Details) board_foot in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) derivative board_foot	Dec. 31, 2018 USD ($)
Debt Securities, Available-for-sale [Line Items]
Equity securities, unrealized gain | $	$ 29,000	$ 43,000
Equity securities, unrealized loss | $	$ 0	$ 105,000
Net Long (Buying)
Debt Securities, Available-for-sale [Line Items]
Non-monetary notional amount (board fee) | board_foot	1,870
Number of derivative instruments held | derivative	17
Short
Debt Securities, Available-for-sale [Line Items]
Non-monetary notional amount (board fee) | board_foot	1,650
Number of derivative instruments held | derivative	15