Investments, Limited Partnerships (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) Investment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Summarized financial information of significant limited partnership investments [Abstract]
Total assets	$ 1,490,131	$ 1,357,016
Total liabilities	1,134,049	938,205
Total partners' capital	32,028	43,586
Regulatory deposits with various insurance departments in United States and Canada, fair value	87,981	86,335
Time certificate of deposit included in short-term investments	$ 1,000
Percentage of fixed income securities invested in investment grade fixed income	90.00%
Number of fixed income investments issued with guarantees | Investment	0
Fixed income investment below investment grade	$ 54,233
Percentage of fixed income investment to total invested assets	6.20%
Number of investments in diversified portfolio including catastrophe bonds | Investment	40
Net unrealized loss position of diversified portfolio	$ 5,202
Limited Partnership Investment, Public Equity and Fixed Markets [Member]
Summarized financial information of significant limited partnership investments [Abstract]
Investment income (loss)	19,507	14,524	$ 13,534
Partnership expenses	9,132	12,861	10,628
Net investment income (loss)	10,375	1,663	2,906
Realized gain (loss) on investments	(37,143)	(15,073)	830
Unrealized appreciation (depreciation) on investments	(48,132)	49,847	46,685
Net increase (decrease) in partners' capital resulting from operations	(74,900)	36,437	50,421
Total assets	241,174	354,709	464,184
Total liabilities	20,020	2,000	14,555
Total partners' capital	221,154	352,709	449,629
Limited Partnerships [Member]
Limited Partners' Capital Account [Abstract]
Commitment to make additional contributions to various limited partnerships	1,317
Summarized financial information of significant limited partnership investments [Abstract]
Investment income (loss)	4,298	623	(5)
Partnership expenses	6,874	2,206	2,426
Net investment income (loss)	(2,576)	(1,583)	(2,431)
Realized gain (loss) on investments	12,314	8,723	7,754
Unrealized appreciation (depreciation) on investments	(65,250)	133,807	(21,002)
Net increase (decrease) in partners' capital resulting from operations	(55,512)	140,947	(15,679)
Total assets	462,058	566,629	448,263
Total liabilities	45,483	30,976	39,988
Total partners' capital	$ 416,575	535,653	408,275
Limited Partnerships [Member] | Limited Partnership Investment, India [Member]
Limited Partners' Capital Account [Abstract]
Number of significant investments in public and private equity markets in India | Investment	1
Significant investment in public and private equity markets in India, value	$ 26,344	29,817
Estimated ownership interest in significant limited partnership investment	6.00%
Income (loss) from limited partnership investment	$ (3,473)	7,665	(1,117)
Limited Partnerships [Member] | Limited Partnership Investment, Public Equity and Fixed Markets [Member]
Limited Partners' Capital Account [Abstract]
Significant investment in public and private equity markets in India, value	$ 14,975	19,380
Estimated ownership interest in significant limited partnership investment	5.00%
Income (loss) from limited partnership investment	$ (4,404)	$ 1,452	$ 2,662