Disclosures About Offsetting Assets and Liabilities (Financial Liabilities and Derivative Liabilities that are Subject to Offset) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Offsetting [Abstract]
Gross amounts of recognized liabilities, Securities lending	$ (299,236)	$ (145,124)
Net amounts presented in the Statement of Financial Condition, Securities lending	(299,236)	(145,124)
Gross amounts not offset in the Statement of Financial Position, Financial instruments, Securities lending	68,274	46,183
Gross amounts not offset in the Statement of Financial Position, Collateral pledged, Securities lending	230,932	98,925
Securities lending, Net amount	(30)	(16)
Gross amounts of recognized liabilities, Securities purchased under agreements to resell	(407,246)	(190,955)
Net amounts presented in the Statement of Financial Condition, Securities purchased under agreements to resell	(407,246)	(190,955)
Gross amounts not offset in the Statement of Financial Position, Financial instruments, Securities purchased under agreements to resell	15,416	17,992
Gross amounts not offset in the Statement of Financial Position, Collateral pledged, Securities purchased under agreements to resell	391,830	172,963
Gross amounts of recognized liabilities	(706,482)	(336,079)
Net amounts presented in the Statement of Financial Condition	(706,482)	(336,079)
Gross amounts not offset in the Statement of Financial Position, Financial instruments	83,690	64,175
Gross amounts not offset in the Statement of Financial Condition, Collateral pledged	622,762	271,888
Net amount	$ (30)	$ (16)