Fair Value of Financial Instruments Level 3 (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at the beginning of the period		$ 10,927	$ 16,435	$ 43,957
Unrealized (loss) gain included in OCI	[1]	(1,733)	(1,057)	(2,608)
Principal payments		(4,351)	(6,019)	(30,602)
Accretion		2,400	1,568	472
Balance at the end of the period		$ 7,243	$ 10,927	$ 16,435

[1] Amount included in “unrealized gain (loss) on available-for-sale investments, net” on consolidated statements of comprehensive income (loss).