Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Consolidated Balance Sheets
Amortized Cost	$ 4,014,397	$ 4,080,663
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, authorized (in shares)	300,000,000	300,000,000
Common stock, issued (in shares)	140,092,172	139,917,150
Common stock outstanding (in shares)	129,827,968	129,928,479
Treasury stock (in shares)	10,264,204	9,988,671