Significant Events and Transactions of the Period - Treasury Shares (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Number of Treasury Shares
Beginning Balance (in shares)	3,903,673	3,500,000
Ending Balance (in shares)	4,533,650
Amount (in thousands of dollars)
Beginning Balance	$ 74,900
Treasury Shares Repurchased	18,239
Ending Balance	$ 79,834
Treasury Stock
Number of Treasury Shares
Beginning Balance (in shares)	3,903,673
Treasury Shares Repurchased (in shares)	(629,977)	1,786,715
Ending Balance (in shares)	4,533,650
Amount (in thousands of dollars)
Beginning Balance	$ 74,900
Treasury Shares Repurchased	4,934	$ (40,080)
Ending Balance	$ 79,834
Treasury Shares Repurchased for RSU Vesting
Number of Treasury Shares
Treasury Shares Repurchased (in shares)	1,258,068
Amount (in thousands of dollars)
Treasury Shares Repurchased	$ 18,239
Treasury Shares Issued for RSU Vesting
Number of Treasury Shares
Treasury Shares Issued for RSU Vesting (in shares)	(628,091)
Amount (in thousands of dollars)
Treasury Shares Issued for RSU Vesting	$ (13,305)