Investments - Narrative (Details) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($) issuer loan	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) issuer loan	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) loan	Dec. 31, 2015 USD ($)
Gain (Loss) on Investments [Line Items]
Number of available-for-sale securities held in an unrealized loss position | issuer	138		138
Total fair value of unrealized loss position securities	$ 437,041		$ 437,041		$ 546,747
Total gross unrealized losses of unrealized loss position securities	7,334		7,334		13,587
Fair value	1,063,490		1,063,490		1,017,478
Carrying value of loan	241,516		241,516		203,526
Unpaid principal on impaired loans	4,400		4,400		7,600
Allowance for credit losses on impaired loans	$ 705	$ 3,700	705	$ 3,700	$ 943	$ 4,300
Number of impaired loans held | loan	4				5
Carrying value of impaired loans	$ 3,600		3,600		$ 6,500
Average recorded investment in impaired bank loans			5,000	5,600
Investment income during period loans were impaired			179	170
Net realized gain (loss) on changes in fair value of impaired bank loans	(42)	913	135	610
Net trading gains (losses)	(1)		1
Net trading gains (losses) still held	(2)		1
Carrying value of collateralized loan obligation held	5,000		5,000
Net realized investment gains	307	1,619	1,354	2,166
Investment income	2,359	522	8,421	1,689
Interest income			1
Bank Holding Company
Gain (Loss) on Investments [Line Items]
Net realized investment gains			409
Investment income				233
Investment in certificate of deposit	1,000
Interest income			172	172
Fixed maturity securities
Gain (Loss) on Investments [Line Items]
Total fair value of unrealized loss position securities	427,464		427,464		518,844
Total gross unrealized losses of unrealized loss position securities	5,895		5,895		10,932
Fair value	982,133		982,133		941,077
Net realized investment gains	(13)	$ 567	29	$ 1,408
Puerto Rico loan
Gain (Loss) on Investments [Line Items]
Carrying value of loan	1,000		1,000		1,700
Unpaid principal on impaired loans	1,200		1,200		2,000
Allowance for credit losses on impaired loans	110		110		177
Oil and gas companies loan
Gain (Loss) on Investments [Line Items]
Carrying value of loan	9,700		9,700
Unpaid principal on impaired loans	2,500		2,500		2,200
Allowance for credit losses on impaired loans	$ 469		$ 469		$ 545
Number of loans | loan	4		4
Bank loan participations gross unrealized losses	$ 1,700		$ 1,700
Number of impaired loans held | loan	2				1
Carrying value of impaired loans	$ 2,000		2,000		$ 1,600
Non-energy sector loan
Gain (Loss) on Investments [Line Items]
Unpaid principal on impaired loans	710		710		3,500
Allowance for credit losses on impaired loans	127		$ 127		$ 221
Number of impaired loans held | loan			1		3
Carrying value of impaired loans	$ 583		$ 583		$ 3,200
BBB- or better | Fixed maturity securities
Gain (Loss) on Investments [Line Items]
Percentage of available for sale securities	99.80%		99.80%
Fair value	$ 2,400		$ 2,400
Aggregate net unrealized gain	$ 204