Derivative Instruments (Consolidated Balance Sheets and Fair Value of the Principal Derivative Instruments) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Derivative Assets
Gross Amounts of Recognized Assets	$ 28,792	$ 28,154
Gross Amounts Offset in the Balance Sheet	2,066	3,829
Net Amounts of Assets Presented in the Balance Sheet	26,726	24,325
Collateral	0	3,601
Net Amount	26,726	20,724
Derivative Liabilities
Gross Amounts of Recognized Liabilities	30,820	8,160
Gross Amounts Offset in the Balance Sheet	5,949	772
Net Amounts of Liabilities Presented in the Balance Sheet	24,871	7,388
Collateral Pledged	21,469	1,473
Net Amount	3,402	5,915
Not Designated as Hedging Instrument | Other assets
Derivative Assets
Gross Amounts of Recognized Assets	24,638	28,090
Gross Amounts Offset in the Balance Sheet	2,066	3,162
Net Amounts of Assets Presented in the Balance Sheet	22,572	24,928
Collateral	0	3,601
Net Amount	22,572	21,327
Not Designated as Hedging Instrument | Other assets | Interest rate futures
Derivative Assets
Gross Amounts of Recognized Assets	139	234
Gross Amounts Offset in the Balance Sheet	(789)	122
Net Amounts of Assets Presented in the Balance Sheet	928	112
Collateral	0	0
Net Amount	928	112
Not Designated as Hedging Instrument | Other assets | Interest rate swaps
Derivative Liabilities
Gross Amounts of Recognized Liabilities	114
Gross Amounts Offset in the Balance Sheet	0
Net Amounts of Liabilities Presented in the Balance Sheet	114
Collateral Pledged	114
Net Amount	0
Not Designated as Hedging Instrument | Other assets | Foreign currency forward contracts, underwriting and non-investment operations
Derivative Assets
Gross Amounts of Recognized Assets	17,824	22,702
Gross Amounts Offset in the Balance Sheet	2,416	2,418
Net Amounts of Assets Presented in the Balance Sheet	15,408	20,284
Collateral	0	0
Net Amount	15,408	20,284
Not Designated as Hedging Instrument | Other assets | Foreign currency forward contracts, investment operations
Derivative Assets
Gross Amounts of Recognized Assets	6,645	1,082
Gross Amounts Offset in the Balance Sheet	439	622
Net Amounts of Assets Presented in the Balance Sheet	6,206	460
Collateral	0	0
Net Amount	6,206	460
Not Designated as Hedging Instrument | Other assets | Credit default swaps
Derivative Assets
Gross Amounts of Recognized Assets	30	37
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts of Assets Presented in the Balance Sheet	30	37
Collateral	0	0
Net Amount	30	37
Derivative Liabilities
Gross Amounts of Recognized Liabilities	332
Gross Amounts Offset in the Balance Sheet	0
Net Amounts of Liabilities Presented in the Balance Sheet	332
Collateral Pledged	332
Net Amount	0
Not Designated as Hedging Instrument | Other assets | Total return swaps
Derivative Assets
Gross Amounts of Recognized Assets		3,744
Gross Amounts Offset in the Balance Sheet		0
Net Amounts of Assets Presented in the Balance Sheet		3,744
Collateral		3,601
Net Amount		143
Derivative Liabilities
Gross Amounts of Recognized Liabilities	17,767
Gross Amounts Offset in the Balance Sheet	0
Net Amounts of Liabilities Presented in the Balance Sheet	17,767
Collateral Pledged	17,767
Net Amount	0
Not Designated as Hedging Instrument | Other assets | Equity futures
Derivative Assets
Gross Amounts of Recognized Assets		291
Gross Amounts Offset in the Balance Sheet		0
Net Amounts of Assets Presented in the Balance Sheet		291
Collateral		0
Net Amount		291
Not Designated as Hedging Instrument | Other liabilities
Derivative Liabilities
Gross Amounts of Recognized Liabilities	30,820	6,342
Gross Amounts Offset in the Balance Sheet	4,460	772
Net Amounts of Liabilities Presented in the Balance Sheet	26,360	5,570
Collateral Pledged	21,469	1,473
Net Amount	4,891	4,097
Not Designated as Hedging Instrument | Other liabilities | Interest rate futures
Derivative Liabilities
Gross Amounts of Recognized Liabilities	1,654	1,545
Gross Amounts Offset in the Balance Sheet	(789)	122
Net Amounts of Liabilities Presented in the Balance Sheet	2,443	1,423
Collateral Pledged	2,443	1,423
Net Amount	0	0
Not Designated as Hedging Instrument | Other liabilities | Interest rate swaps
Derivative Liabilities
Gross Amounts of Recognized Liabilities		50
Gross Amounts Offset in the Balance Sheet		0
Net Amounts of Liabilities Presented in the Balance Sheet		50
Collateral Pledged		50
Net Amount		0
Not Designated as Hedging Instrument | Other liabilities | Foreign currency forward contracts, underwriting and non-investment operations
Derivative Liabilities
Gross Amounts of Recognized Liabilities	9,700	3,808
Gross Amounts Offset in the Balance Sheet	4,810	28
Net Amounts of Liabilities Presented in the Balance Sheet	4,890	3,780
Collateral Pledged	0	0
Net Amount	4,890	3,780
Not Designated as Hedging Instrument | Other liabilities | Foreign currency forward contracts, investment operations
Derivative Liabilities
Gross Amounts of Recognized Liabilities	440	939
Gross Amounts Offset in the Balance Sheet	439	622
Net Amounts of Liabilities Presented in the Balance Sheet	1	317
Collateral Pledged	0	0
Net Amount	1	317
Not Designated as Hedging Instrument | Other liabilities | Equity futures
Derivative Liabilities
Gross Amounts of Recognized Liabilities	813
Gross Amounts Offset in the Balance Sheet	0
Net Amounts of Liabilities Presented in the Balance Sheet	813
Collateral Pledged	813
Net Amount	0
Designated as Hedging Instrument | Other assets | Foreign currency forward contracts, investment operations
Derivative Assets
Gross Amounts of Recognized Assets	4,154	64
Gross Amounts Offset in the Balance Sheet	0	667
Net Amounts of Assets Presented in the Balance Sheet	4,154	(603)
Collateral	0	0
Net Amount	4,154	(603)
Designated as Hedging Instrument | Other liabilities | Foreign currency forward contracts, investment operations
Derivative Liabilities
Gross Amounts of Recognized Liabilities	0	1,818
Gross Amounts Offset in the Balance Sheet	1,489	0
Net Amounts of Liabilities Presented in the Balance Sheet	(1,489)	1,818
Collateral Pledged	0	0
Net Amount	$ (1,489)	$ 1,818