Fair Value - Rollforward of Level 3 assets and liabilities (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		$ 84,889	$ 54,238	$ 86,771	$ 54,633
Total gains or (losses) (realized/unrealized) - included in earnings		(198)	3,708	(987)	4,672
Total gains or (losses) (realized/unrealized) - included in other comprehensive income		(320)	0	(176)	0
Purchases, issuances, sales and settlements
Purchases		393	4,935	393	4,935
Issuances		0	0	0	0
Sales		(5,077)	(26,242)	(5,077)	(26,242)
Settlements		(989)	(69)	(2,226)	(1,428)
Transfers in and/or out of Level 3		0	0	0	0
Balance at end of period		78,698	36,570	78,698	36,570
Contingent consideration liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		(62,449)	(125,544)	(60,996)	(122,350)
Total gains or (losses) (realized/unrealized) - included in earnings		(1,481)	(3,441)	(2,934)	(7,087)
Total gains or (losses) (realized/unrealized) - included in other comprehensive income		0	0	0	0
Purchases, issuances, sales and settlements
Purchases		0	0	0	0
Issuances		0	0	0	0
Sales		0	0	0	0
Settlements		0	71,739	0	72,191
Transfers in and/or out of Level 3		0	0	0	0
Balance at end of period		(63,930)	(57,246)	(63,930)	(57,246)
Available for sale | Structured securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	[1]	5,413	10,637	5,927	11,289
Total gains or (losses) (realized/unrealized) - included in earnings	[1]	3	3,072	4	3,779
Total gains or (losses) (realized/unrealized) - included in other comprehensive income	[1]	(4)	0	(8)	0
Purchases, issuances, sales and settlements
Purchases	[1]	0	0	0	0
Issuances	[1]	0	0	0	0
Sales	[1]	(5,003)	(13,640)	(5,003)	(13,640)
Settlements	[1]	(33)	(69)	(544)	(1,428)
Transfers in and/or out of Level 3	[1]	0	0	0	0
Balance at end of period	[1]	376	0	376	0
Available for sale | Corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		9,152	18,601	9,460	18,344
Total gains or (losses) (realized/unrealized) - included in earnings		0	636	0	893
Total gains or (losses) (realized/unrealized) - included in other comprehensive income		(316)	0	(168)	0
Purchases, issuances, sales and settlements
Purchases		393	4,935	393	4,935
Issuances		0	0	0	0
Sales		0	(12,602)	0	(12,602)
Settlements		(456)	0	(912)	0
Transfers in and/or out of Level 3		0	0	0	0
Balance at end of period		8,773	11,570	8,773	11,570
Fair value option | Corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		11,872	0	12,217	0
Total gains or (losses) (realized/unrealized) - included in earnings		(537)	0	(612)	0
Total gains or (losses) (realized/unrealized) - included in other comprehensive income		0	0	0	0
Purchases, issuances, sales and settlements
Purchases		0	0	0	0
Issuances		0	0	0	0
Sales		0	0	0	0
Settlements		0	0	(270)	0
Transfers in and/or out of Level 3		0	0	0	0
Balance at end of period		11,335	0	11,335	0
Fair value option | Other investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		58,452	25,000	59,167	25,000
Total gains or (losses) (realized/unrealized) - included in earnings		336	0	(379)	0
Total gains or (losses) (realized/unrealized) - included in other comprehensive income		0	0	0	0
Purchases, issuances, sales and settlements
Purchases		0	0	0	0
Issuances		0	0	0	0
Sales		(74)	0	(74)	0
Settlements		(500)	0	(500)	0
Transfers in and/or out of Level 3		0	0	0	0
Balance at end of period		$ 58,214	$ 25,000	$ 58,214	$ 25,000

[1] Includes asset backed securities, mortgage backed securities and commercial mortgage backed securities.