Variable Interest Entities (Narrative) (Details) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2020 USD ($) entity	Dec. 31, 2019 USD ($) entity
Variable Interest Entity [Line Items]
Liabilities, contingent liabilities, or guarantees		$ 6,559,627	$ 7,952,868
Number of VIE's that hold junior subordinated notes. | entity		2	2
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Liabilities, contingent liabilities, or guarantees		$ 0	$ 0
U.S. Insurance JV [Member]
Variable Interest Entity [Line Items]
Unfunded commitments	[1],[2]	$ 817	$ 817

[1] Not applicable.

[2] The U.S. Insurance JV invests in debt issued by small and medium sized U.S. and Bermuda insurance and reinsurance companies.