Investments - Narrative (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) issuer loan	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) issuer loan	Sep. 30, 2016 USD ($)	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	146		146
Total fair value of unrealized loss position securities	$ 469,005		$ 469,005		$ 546,747
Total gross unrealized losses of unrealized loss position securities	$ 6,602		$ 6,602		13,587
Percentage of available for sale securities	12.20%	10.00%	12.20%	10.00%
Fair value	$ 1,095,523		$ 1,095,523		1,017,478
Unpaid principal on impaired loans	4,000		4,000		7,600
Carrying value of loan	245,741		245,741		203,526
Allowance for credit losses on impaired loans	1,500	$ 2,000	$ 1,500	$ 2,000	$ 943	$ 4,300
Number of impaired loans held | loan			4		5
Carrying value of impaired loans	2,400		$ 2,400		$ 6,500
Average recorded investment in impaired bank loans			4,500	8,000
Investment income during period loans were impaired			231	237
Net realized gain (loss) on changes in fair value of impaired bank loans	729	663	594	53
Net trading gains (losses) still held	2
Net trading gains (losses)			0
Carrying value of collateralized loan obligation held	5,000		5,000
Net realized investment (losses) gains	(171)	210	1,183	2,376
Investment income	2,316	4,779	10,737	6,467
Bank Holding Company
Gain (Loss) on Investments [Line Items]
Net realized investment (losses) gains			409
Investment income				266
Investment in certificate of deposit			1,000
Interest income			2	3
Fixed maturity securities
Gain (Loss) on Investments [Line Items]
Total fair value of unrealized loss position securities	456,245		456,245		518,844
Total gross unrealized losses of unrealized loss position securities	5,089		5,089		10,932
Fair value	1,016,818		1,016,818		941,077
Net realized investment (losses) gains	374	$ 70	403	$ 1,477
Puerto Rico loan
Gain (Loss) on Investments [Line Items]
Unpaid principal on impaired loans	807		807		2,000
Carrying value of loan	0		0		1,700
Allowance for credit losses on impaired loans					177
Oil and gas companies loan
Gain (Loss) on Investments [Line Items]
Unpaid principal on impaired loans	2,500		2,500		2,200
Carrying value of loan	9,700		9,700
Allowance for credit losses on impaired loans	$ 500		$ 500		$ 500
Number of loans | loan	4		4
Bank loan participations gross unrealized losses	$ 1,900		$ 1,900
Number of impaired loans held | loan			2		1
Carrying value of impaired loans	1,900		$ 1,900		$ 1,600
Non-energy sector loan
Gain (Loss) on Investments [Line Items]
Unpaid principal on impaired loans	708		708		3,500
Allowance for credit losses on impaired loans	209		$ 209		$ 221
Number of impaired loans held | loan			1		3
Carrying value of impaired loans	$ 500		$ 500		$ 3,200
BBB- or better | Fixed maturity securities
Gain (Loss) on Investments [Line Items]
Percentage of available for sale securities	99.10%		99.10%
Fair value	$ 9,000		$ 9,000
Aggregate net unrealized gain	$ 248