Fair Value Measurements - Changes in Level 3 Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset beginning balance	$ 168	$ 158	$ 158	$ 157
Asset, gain (loss) included in earnings	(1)	31	9	32
Asset ending balance	167	189	167	189
ARS Purchase Commitments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset beginning balance				0
Asset, gain (loss) included in earnings				905
Asset ending balance		905		905
Interest Rate Lock Commitments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset beginning balance		13,453		9,161
Liability beginning balance				923
Asset, gain (loss) included in earnings		53		4,345
Liability, gain (loss) included in earnings				923
Sales and settlements		13,453		13,453
Asset ending balance		53		53
Liability ending balance		0		0
Auction Rate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset beginning balance	107,170	89,101	84,926	86,802
Asset, gain (loss) included in earnings	(161)	(417)	983	3,157
Asset, purchases	25	2,000	22,075	13,775
Sales and settlements	(250)	(675)	(1,200)	(13,725)
Asset ending balance	106,784	90,009	106,784	90,009
Auction Rate Securities Purchase Commitment [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset beginning balance		911	849
Liability beginning balance	254	142	645	1,369
Asset, gain (loss) included in earnings		(6)	(849)
Liability, gain (loss) included in earnings	(22)	(134)	369	1,093
Asset ending balance	0	905	0	905
Liability ending balance	276	276	276	276
Municipal Bonds [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset beginning balance	36	25	44	81
Asset, gain (loss) included in earnings	(1)	0	(9)	6
Sales and settlements	0	0	0	(62)
Asset ending balance	$ 35	$ 25	$ 35	$ 25