Significant Events and Transactions of the Period - Treasury Shares (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020
Number of Treasury Shares
Beginning Balance (in shares)		3,903,673		3,500,000	3,903,673
Ending Balance (in shares)	5,589,408				5,589,408
Amount (in thousands of dollars)
Beginning Balance		$ 74,900			$ 74,900
Treasury Shares Repurchased	$ 14,861	$ 18,239
Ending Balance	$ 90,714				$ 90,714
Treasury Stock
Number of Treasury Shares
Beginning Balance (in shares)		3,903,673			3,903,673
Treasury Shares Repurchased (in shares)	(1,055,758)	(629,977)	553,435	1,786,715
Ending Balance (in shares)	5,589,408				5,589,408
Amount (in thousands of dollars)
Beginning Balance		$ 74,900			$ 74,900
Treasury Shares Repurchased	$ 10,880	$ 4,934	$ (12,515)	$ (40,080)
Ending Balance	$ 90,714				$ 90,714
Treasury Shares Repurchased for RSU Vesting
Number of Treasury Shares
Treasury Shares Repurchased (in shares)					2,525,492
Amount (in thousands of dollars)
Treasury Shares Repurchased					$ 33,100
Treasury Shares Issued for RSU Vesting
Number of Treasury Shares
Treasury Shares Issued for RSU Vesting (in shares)					(839,757)
Amount (in thousands of dollars)
Treasury Shares Issued for RSU Vesting					$ (17,286)